GENRTF appears to be part of the CHARMMing site, and I believe it's main utility lies in creating residue definitions to facilitate QM/MM calculations. There is a specific forum for questions about CHARMMing (and it is not this one).
If you wish to add new molecules to CHARMM, see the Parameter forum, esp. the information on CGenFF and the ParamChem site. GENRTF is NOT suitable for this purpose.