The Bones star wrote on Twitter that he wants to play a love interest for Mary (Michelle Dockery) in the show.
In the Christmas special, which aired in the US on Sunday, the Earl of Grantham (Hugh Bonneville) suggests that Mary travels to America to avoid a scandal and meet an "American cowboy".
"I'd like to be that American cowboy who shakes things up at Downton Abbey," Boreanaz tweeted. "I'm open for a visit Mr. Fellowes."
However, Mary eventually got engaged to Matthew Crawley (Dan Stevens).
Boreanaz also praised Downton Abbey, calling it a "great series" with "great production value".
Oscar-nominated actress Shirley MacLaine has already signed up for a role in Downton Abbey's third season, while War Horse actor Matt Milne will join the cast as a new footman.