Actually, I've gotten it to work. I don't even need A441's tool anymore. What I do:
- Open Wintex. Copy the Texture1/Pnames lump out of SRB2.srb and into my wad
- Open XWE, with my wad, and open pnames. Add my textures to the bottom of the list (Make sure you get the number correct!). Save.
- Open Texture1 and edit textures (create new textures, add patches, resize textures, etc), then save that
And... surprisingly, it works!