I'm not sure what you mean by "can't be converted of course." Does the Blender script reject loading files with other extensions? Because if so, I would say that's a bug. Or does it use parts.lst and only support library files that are in there? Or is something else going on?
The .ums files that UnMirrorStuds creates are standard LDraw files. And the .ums version of the model itself references these files. (If the model has sub-models, .ums versions of those are created if needed, referencing .ums files where appropriate. I don't remember off-hand what the .um2 files are. I think they're for mirrored sub-models that themselves contain both mirrored and non-mirrored studs.) I suppose the UnMirrorStuds program could be updated to append -ums to the base filename instead of changing the extension to .ums if the extension really is causing problems, but I don't understand why that would be the case.
The .ums files that UnMirrorStuds creates are standard LDraw files. And the .ums version of the model itself references these files. (If the model has sub-models, .ums versions of those are created if needed, referencing .ums files where appropriate. I don't remember off-hand what the .um2 files are. I think they're for mirrored sub-models that themselves contain both mirrored and non-mirrored studs.) I suppose the UnMirrorStuds program could be updated to append -ums to the base filename instead of changing the extension to .ums if the extension really is causing problems, but I don't understand why that would be the case.