Easiest way would just be to use the closest equivalent; if you have Fire and Ice use elemental champion for CC for elemental warrior, similarly for ChCo semi priest for CC Champion; if you only have the basics - paladin for CC champion, magent for CC venturer, etc.
Harder way would be to use any of the conversion documents in the vault or if you can find the old Irregular Realms article from the guild companion to build equivalent versions.
One thing to remember is that RM2/RMC spell lists tend to be more powerful than RMSS.