As a skydiver, I have jumped from many different types of aircraft. Most have been specially modified for the sport. Are there requirements outside of the mechanical modifications for an aircraft to be used as a jump plane? Is there some kind of certification for jump planes in the United States?
Not really. You use whatever old beater airplane you can get your hands on with a suitable configuration. And you may not really need to do a mod like a jump door STC if you're willing to fly with the door removed and the plane is legal to fly that way.
When I was jumping in the late 70s it was from a Cessna 172 that was flown without a door, then later the owner installed a top hinged swing up door available as an STC kit so we wouldn't freeze on the ride up. Other than that it's just take the seats out.