It's important to keep in mind the sheer speed of the Islamic conquests. Muhammad left Mecca for Medina in 622CE. By 629, Islam had become a powerful religion in the Arabian peninsula Islamic armies clashed in the Levant region in the Battle of Mu'tah. The Muslims lost in 629, but in 634 they returned and by 637 they had taken Jerusalem and successfully conquered the region. This leaves relatively little time for Muslims to establish themselves in the region prior to the Islamic conquest. This doesn't mean however that there would not have been any. Three ways that Muslims would have been in the Levant region prior to the conquests would have been: commercial contacts with the Arabian elite, the conversion of tribes that lived near the border of the Arabian Peninsula and the Levant, and pilgrimage to Jerusalem.

As the Levant was a mixing ground between Arab, Hellenistic, and Syriac culture, there was some Muslim presence from Arabian tribal elites. Abu Sufyan ibn Harb, the leader of the Quraysh tribe of Mecca (Muhammad's own tribe that ousted him and fought against him), owned property in Syria. Although he fought against Muhammad, he ended up converting to Islam and fought as a commander in the various expansionist Islamic wars. I have not read anything stating that Abu Sufyan returned to Syria in between his conversion and the conquests, but the fact that he owned property there hints at the possibility other Muslims may have made the journey.

Also, many of the tribes at the Arabian Peninsula-Levant border had converted to Islam prior to the conquest of the Levant proper (~630CE). Among these are the Banu 'Udhra and the Banu Sa'd Hudhaym, both nomadic tribes, and the tribe of Balī. This last one is notable as they were historically staunchly anti-Muhammad; it was members of this tribe that led the Arab auxiliaries to the Byzantine Army that defeated Muhammad at the Battle of Mu'tah. The conversions of these tribes, whether for actual faith or for political gain, means that Muslims would have been known in the region.

Finally, it is also likely that Muslims ventured to Palestine before the conquests because of the centrality of Jerusalem to the Islamic faith. It was, and remains today, a popular pilgrimage spot.

I drew most of this information from Fred Donner's book The Early Islamic Conquests which has an extremely detailed account of the conquest of Syria.