You are encouraged both to modify any of the existing game designs and to develop your own. Chapter 3, "Designing Games with Xconq", includes a comprehensive tutorial on how to do this. The version 7 game design language (GDL) is better-designed and more robust than the machinery in version 5, so if you've been discouraged by mysterious problems before, you might want to try designing with version 7 instead.
The ftp server ftp.cygnus.com usually has the latest version of
Xconq, and possibly other contributed material, in the directory
pub/xconq. The directory pub/games/xconq on
ftp.uu.net has long had Xconq material, although it is
generally for version 5. Other servers also have copies of Xconq.
See your local Internet wizard, or buy one of the many excellent
Internet guides if this is not enough information for you.