Let's consider he Hunnic army that invaded Gaul in 451 and engaged in the famous battle of Chalons/Catalaunian Fields/locus Mauriacus. This was composed of numerous peoples under Attila's sway and a list of these peoples is found in the Panegyric to Avitus which was composed by Sidonius Apollinaris; of note is that Sidonius peppered his list with names of fictional tribes and outdated names but nonetheless he was writing only about six years after the Hunnic invasion and so he had first hand information regarding the situation. Among the real groups he names include the Gepidae, Sciri and the Burgundians (the Burgundians had been nearly annihilated in a Hunnic campaign and chroniclers attributed their survival to divine intervention).

The Huns certainly did use conquered peoples in their army as this list attests and these peoples would often be ruled by picked men (logades). The leader of the Sciri was a Hun by the name of Edeco, who not only was Attila's picked man but also one of his bodyguards. The Hunnic army was also bolstered by the presence of Roman deserters who probably provided the Huns with their siege knowledge and made the steppe people even more formidable than usual. Attila also had Roman secretaries and confidants such as Orestes, who would become the father of the last Roman emperor. So it is within possibility that the Huns might incorporate a Frank into their army (indeed, the target of Attila's Gallic campaign might not have been the Visigothic kingdom but rather to settle a succession dispute among the Franks in his favor - see the works by Hyun Jin Kim for this view).

Sources:

Kim, Hyun Jin. The Huns, Rome and the Birth of Europe

Kim, Hyun Jin. The Huns

Maenchen-Helfen, Otto. The World of the Huns: Studies in Their History and Culture

A fairly interesting source on this is Priscus' account of an embassy from the Eastern emperor to the Huns in 448/9. While at Attila's camp Priscus meets a Greek merchant who had been captured by the Huns at the sack of Viminacium(441), and then rose to fairly high status (the source I linked doesn't have page numbers or anything, but the relevant bit is in the sixth paragraph down, the one that begins: 'When Attila entered the village').

It's worth noting that they then go on to have a nicely structured debate about the relative merits of the Roman and Hunnic governments, so the incident may well be largely fictional, but there's some interesting descriptions of the ethnic makeup of the 'Hunnish' army in the surrounding passages anyway.