Is there any viable way to transfer a Doff from 1 character to an alt?
If they aren't bound, yes. I assume that's what you're asking. Just drag them from the roster to your mail slot and send them off. I've done this once from an alt to a main (a blue that I wanted and which wasn't bound to said alt).
Right, I forgot to mention what Kathy said - you can't mail or trade Doffs that are bound to the character (they have a little "lock" icon next to their trait icons), and you also can't mail/trade ones on assignment or in sickbay.