Dragon Age 2 DLC transfer into DAO 3?

Nothing is known about DA3 or the role Sebastian will play. A dev did say that his story isn't over, though this could just mean he shows up in an expansion or future DLC. But, I really doubt your decision at the end of the game will change anything. If they make Sebastian a part of future games they will likely establish a permanent canon for him to account for people who didn't buy the DLC.