Mixing expansions from different core sets

[iYam]

I know you can mix all core sets you'd like, as long as you can manage the deck size, and also add in expansions for their respective core sets, so for example: Super Munchkin + Fantasy Munchkin + 2: Unnatural Axe.
My question is; can one mix an expansion of a core set they don't have?
For example: Munchkin Pathfinder + 4: Need for Steed.