The problem is that the UPE is not all of Platform Builder. It's just the game-related stuff, not the builder-related stuff. So you'll be able to edit game stuff, but you can't edit builder stuff. Because the UPE is for Game Maker, all of the "building" is done in Game Maker. So you could import a pbx file, or you could literally just build the game from the ground up using Game Maker.