@Toveran What are you trying to do? Are you wanting to play your gta5 without building collisions? Or are you wanting to spawn a vanilla building somewhere for a mod you are making? If it is the first option, you can just delete the collisions through OpenIV, but you're going to hate the results. Here's a quick video of that method and the results.
Correct method would have you spend A LOT of time picking and choosing what collisions to remove and you will need to use 3ds to remove them. Collisions are layered in many ways in many files for one area. You aren't going to fiddle with them in Menyoo or Map Editor. If you are wanting to place a collisionless building for a mod you're making, just use CodeWalker to make the mod.
No need to try to convert a xml to a xml, that doesn't make sense. Either import the xml from the file location or just throw the xml on your desktop, then CodeWalker has an option in the Tools tab inside the project window to "Import Menyoo XML", it really should just say xml files. Doesn't matter if it's a map editor or menyoo xml. Should be able to save the project as a ymap through CW when you're done making the mod.
ps. the only reason I did not fall through the map when I went through the building to inside my interior that you could not see, is that I still had my mod's collision there. The drawable stuff disappears because of going through the building and not a portal.