From ESPN's rules
"NOTE: Roster settings can be edited until the draft begins. After the draft is complete roster settings can not be changed. You can not add slots after your league has drafted. You must reset the draft in order to do so."
What about letting the 10 current teams claim 3 keepers each that are LOCKED IN and the expansion teams can't touch? Then the rest of the current teams, including their other 2 keepers, are up for grabs, from which the two new teams can each pick their 5 keepers. Any players that aren't picked by the expansion teams stay on their original teams, whose managers will again have exclusive rights to claiming them as one of their last 2 keepers.
Does that make sense? Though I suppose it would be problematic to add a rule like that after letting the current teams operate under the assumption they had their pick of their 5 keepers no matter what. Ideally, you would have had a contingency plan for expansion teams set in place in your league rules.
Other than that, I'm trying to think of how you could accomplish what you're trying to do... I use ESPN also and appreciate how little flexibility their keepers system has unfortunately. I'll try to think of a better solution and get back to you...
But to answer your main question, no, you cannot edit roster positions after the draft.