If you have a look at the CB sources
http://svn.berlios.de/wsvn/codeblocks/trunk/src/plugins/compilergcc/compilerGNUARM.cpp
http://svn.berlios.de/wsvn/codeblocks/trunk/src/plugins/compilergcc/compilerGNUARM.h
you see an example how a GCC compiler should look like. After creating such files for 68HC12 you have to add the cpp, h file to the CB project file and in addition
http://svn.berlios.de/wsvn/codeblocks/trunk/src/plugins/compilergcc/compilergcc.cpp
add an include directive for the corresponding h file.