You can try to merge the groups.

Or, you can select the surfaces that you want to give a different texture, copy them and delete them, then switch to the group with the other texture, and paste the surfaces.

Don't know, whether this works, though.

http://www.puppenheim.org/
http://www.wuerfels-welt.de
Old Game Tutorial: http://www.puppenheim.org/MITutorial.zip
Free Game Editor: http://puppenheim.org/AlienalsWeihnachtsmann.zip
http://www.puppenheim.org/blender_starting_main_settings.txt