It's never that easy, even porting maps from GES alpha to beta will need some "hammering" and this is the same engine.
Sure you can simply throw it them into your maps-folder but certain things like weaponspawns or ladders won't work.
Now imagine you wanna port a DM map from a EP1 game to well TF2.
So the easiest way to have a GE map in TF2 is to start from scratch with it, coz you'll need another size for doors, windows etc. and you'll also need other textures to fit the style of the game.