Exient's XGS Engine's a mess to modify, XGM is models/meshes(?), XGSAT I've never seen unless you made a typo for XGSA (Which is the EXACT same format as XGM afaik), XGSF is fonts and XMAT is just shader data compiled (not really). Afaik, XGM's cannot be modified, yet, but can be viewed. Here's some threads that talked about most of the formats you said to help.
XGM (NOT YET EDITABLE): viewtopic.php?f=5&t=8689 viewtopic.php?f=7&t=9049 viewtopic.php?f=5&t=12721
XGT (Editable with manual byte manipulation): viewtopic.php?f=7&t=8000
XLC: (Viewable/Editable[?]) viewtopic.php?f=12&t=11343
XAL (NOT YET EDITABLE): viewtopic.php?f=9&t=12488
ATLAS (NOT YET EDITABLE): viewtopic.php?f=7&t=12375