I bought The Witcher 3 + Expansion pass, and now i'm wondering - will i be able to add Blood and Wine to my current playthrough when it releases, or will i have to start a new game to participate in the DLC?

It's not out yet, I don't think anyone knows but my guess it's going to work like Hearts of Stone: it will have its own main quest and side quests that that can be played in your current playthrough but with the option to create a pre-defined Geralt with some fitting gear and the appropriate level without earlier saves.