Hmm, tricky... if you don't mind converting them, that would be a much easier route to go down!
Calibre is a pretty neat program for this purpose...
I'm currently ripping things like epubs to their raw HTML and then PalmDoc'ing (For texty ones) or iSilo'ing (If they have pics) them. (I'm gonna be screwed when I move off PalmOS as I will lose a lot of my ebook book library!)
Sorry for the late response but if you get the Mobipocket desktop (I use xp version) the desktop can convert EPUB documents. Put the epub document in your computer go to the book list on mobipocket desktop and press import. Click the epub document and the desktop will convert it and put it in your mobipocket library for import to Palm devices or manually find its location and put it in your memory card.
Yeah mobipocket hasn't update for a long while but it is also great to import pdf files into the palm library.
Th-55 U, T/X, TG-50, Handspring Deluxe (3) and PSP user.