> Well I've looked at everything. The only way to get a new theater into Yuri's Revenge seems to be to create a new sort of .tmp style file (like .ubn, or .*** whatever), but the game will not acknowlege that the new sort of file is in fact a kind of .tmp file.

You need to update certain INIs.

> Looks like I'm going to have to simply resort to modifying an existing type, like the .des (boy, am i gonna miss deserts).
> I sure wish there was some way to work around that, how does it identify that those .ubn files are to be treated as .tmp files for the New Urban theater anyway? Oh, well. I tried, and you made the software that let me try. I don't want to live my life without failing at at least one thing! Thanx for making the mix editor anyway, it will help me.

You might want to have a look at the unofficial terrain expansion.