I presume that you are asking about the 'A' model airframe with the rear bench seat.
This would therefore imply recertification from 4 to 5 occupants which may not be viable or even acceptable.
In this case it would be simpler to purchase a 'B' (or 'C' model if you can find one) which are certificated up to 6 occupants.
I can't remember the exact ins and outs but, in certifying to more than 4 seats (or possibly more than 5) the aircraft must have exits on both sides. On the Comanche this was achieved by modifying and shifting the baggage door to the left side.
There may be STC for rear seat shoulder restraints (for all variants) but that's about as much as I know.
This organisation will know more
http://www.comancheflyer.com/