Out of interest, is the intent to transfer custom map data from one map to other melee maps, or is it to transfer custom races from a variety of sources to compile them into one map (for example, taking multiple of Wa666r's custom races and compiling them together similarly to how Ultimate Battle did)?
If it's the latter, then the process will become a lot more involved, although I have heard that transferring object data into a campaign file can help make the process of copying and pasting data into the same map a whole lot easier. I do not know how Triggers would be handled in these events, though, without having to handle them individually on a case-by-case basis.