Really? That does not happen for me. How are you loading them up? I usually create my empire with USM loaded, save it to file, then use it in a PBW game, and it has always worked fine so far.
Well, it does add yours, but appends all of them at the end of the default list. Which is annoying, because one would have to use that delete, select, delete, select.
It appears as though the default design names are built into the code... !?