I THINK - Create a new category with standard ULX commands, and by default, XGUI should pick up on it.
Make sure you use ULX 'help' object in your comman setup, and we strongly recommend using 'default' variable in your parameter setup.
To mu knowledge, there is no XGUI documentation except for comments in the code itself.
ULX is reasonably commented within the code itself, and, much of it relies on ULib commands, which are all documented here -
http://ulyssesmod.net/docsI wrote a decently commented ULX command structure tutorial for how to convert our old 2-3 year command setup to new (now 1-2 year old) command structure.
See half way through (NEW STUFF HERE part) at
http://forums.ulyssesmod.net/index.php/topic,4464.msg17838.html