You are encouraged both to modify any of the existing game designs and to develop your own. The manual "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 sources.redhat.com usually has the latest
version of Xconq, along with 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.