As far as I can recall from all of my (fairly extensive) reading of Frankish sources there is not a single example of this in the annals.

There may be some vitae (accounts of the lives of saints) or miraculae (accounts of the miracles of deceased saints) but I can't recall any, frankly.

Monasteries and monks seem to be the primary ecclesiastical group described rather than nuneries, and the sources typically don't give extensive details, just noting that many people were carried off. Certainly both men and women were taken captive, either to be ransomed or to be sold as slaves in Scandinavia or (more often) into the Islamic world. But the idea of whole scale "woman stealing" seems to be a later invention.

In support of this is a great article by Simon Coupland “The Vikings on the Continent in Myth and History.” History 88, no. 290 (2003): 186–203.

He points out that while later accounts of the Vikings, and laterideas of them, focus heavily on the idea of rape, there seems to be very little description of this act in the contemporary sources.

As he writes, "the Vikings were not merely ‘long-haired tourists who occasionally roughed up the natives’. To fall into their hands must have been a terrifying experience. Yet the contemporary evidence indicates that they were no worse than the Franks in their brutality, and although they almost certainly did commit rape as well as murder and torture, they were not thought of as notorious rapists. What is more, they would often spare their prisoners’ lives if there was the chance of a ransom, as occurred at both Nantes and St Bertin: ‘By the merits of the monastery’s patrons those who had been taken prisoner soon returned, all happy and rejoicing.’ 65 Here, then, is clear evidence both of Viking cruelty and also of the stereotype of bestial brutality distorting both medieval and modern perceptions of the Scandinavian invaders." (197)

"Women's Roles in the Middle Ages" By Sandy Bardsley mentions Viking attacks on two convents in 870 at Barking and Ely respectively. There's no mention of mass rape or kidnapping during these raids in any of the sources I've been able to find.

According to this page: http://wychwood.wikidot.com/history-nuns

"The prolific eleventh century Flemish hagiographer Goscelin of St Bertin writes that the nunnery at Barking had been burnt together with the nuns and their abbess by the Danes in the same year in which King Edmund of East Anglia was martyred by the Danes, namely 870."

Which suggests that they were killed rather than carried off. There's very little other source material on attacks of this type that I'm aware of and certainly no explicit mention of nun's being carried off en-masse as the trope would suggest.

The Great Viking Army invaded England in 865. During the next few years they ravaged the country. When they marched through East Anglia in 869-870, their work must have been particularly effective. Practically no records survive from East Anglia before this date, suggesting that the viking army systematically plundered church institutions that would have otherwise preserved their charters and other documents in their archives. If ever there were a time to find a nun among viking captives, this would be it.

Sometime during the year 870 the bishop Rimbert traveled north from his Archdiocese of Hamburg-Bremen as part of his missionary duties. He passed through the Danish trading town of Hedeby, where slaves from the North Sea were shuffled off into the Baltic and thence into the unknown. Many of them probably were trucked south and east along the Russian riverways as famously described by Ibn Fadlan a generation later. There, among the fettered masses, the Bishop Rimbert must have heard many voices cry out for compassion, but above the rest, he heard one that had been trained to sing the psalms. Here, if nowhere else, is certainly an account of a nun enslaved:

When Rimbert came to a that part of the Danish lands where he had built a new Christian church in the place called Schleswig (Hedeby), he saw a number of Christians bound together and drawn into captivity. Among these was a certain pious woman, who saw him from afar. She bent her knee and bowed her head, seemingly as much to venerate the holy man as to pray for her own redemption.

So that he would recognize her as a Christian, she raised her voice and began to chant psalms. The bishop was moved to compassion; by divine grace he heard her prayers ring out amidst her tears. For the sake of her prayers he soon moved to break the fetters which bound her neck, but lest she flee, the pagans had also bound her hands.

The holy bishop, then moved to pity, began to offer the pagans the various valuable things he had for her. But under no condition would they be accede unless he gave them also his horse, which he was accustomed to using as his special mount. But refusing nothing, he immediately leapt down from his horse and prepared to give them all that he had for that captive, and he bought her redemption and allowed them to depart with whatever they wanted.

Twice, as we say, did this deed leave memories worthy of praise, since she broke her bonds by the virtue of prayer and since this sign showed a work of his mercy. (Vita Rimberti, ch. xvii).

It's worth noting that we have no substantiating information for the Life of Rimbert, except for the fact that the historian Adam of Bremen referred to it almost two-hundred years after Rimbert's death. Adam was a consummate archival researcher, and I trust his implicit judgment that the text looked authentic. For Adam to accept its authority, it was probably written in a late ninth-century hand and bound with other texts from the period shortly after Rimbert's death.

So even if we skeptically dismiss the accuracy of the events described in the Life of Rimbert, it's safe to say that there was someone at Hamburg-Bremen around the year 900 who could assume that nuns were for sale in the Baltic slave markets. Both the author and his audience lived on the fringe of this Baltic world, and as members of a missionary diocese, they often traveled through it. Rimbert himself may have never ransomed this captive nun, but since the account is plausible, there were most certainly nuns crying out for redemption on the docks of Norse Hedeby.