Short answer is that it is very difficult. Not impossible, certainly, but the toolkit is based on the assumption of sequential turns, so this is only something you should attempt when you feel you have a complete understanding of how the toolkit works. If you still want to try this, the general approach would be to delay running actions in the action manager until all moves have been selected and then running the actions while enabling the bWithPrevious boolean. That is just the first step, though, and it would certainly require heavily rewriting the ability system and turn manager. It might be complicated enough that it would be a better idea to build a project from scratch and port over the ATBTT functionality you need as you go along.
Ok, I see. I misunderstood. To get this effect go to the CreateBaseEdges function in BP_GridManager and change the default move cost values for diagonal directions from 1 to 2. Like so:
