THE ACFT WAS ON A SPORT PARACHUTE FLT TO CARRY 24 PARACHUTISTS TO 12,500' OVER A DROP ZONE (DZ) FOR A MASS JUMP. IT WAS IN A CARGO CONFIGURATION WITH NO PASSENGER SEATS, BUT IT HAD 1 JUMP SEAT. THERE WERE 24 SEAT BELTS ON THE FLOOR OF THE CARGO AREA. ALSO, THE CABIN DOOR HAD BEEN REMOVED & AN UNAPPROVED STEP & 4 HANDHOLDS WERE INSTALLED OUTSIDE & FORWARD OF THE CABIN DOOR FOR PARACHUTISTS. THE USUAL JUMP-RUN PROCEDURE WAS FOR THE ACFT TO BE SLOWED TO 95 TO 100 KTS, EXTEND THE GEAR & APCH FLAPS & REDUCE POWER ON THE LEFT ENG. THE JUMPERS REMAINED IN POSITION WITHOUT USING SEAT BELTS UNTIL APCHG THE DZ, THEN 2 MOVED OUTSIDE THE ACFT WHILE OTHERS MOVED CLOSE TO THE DOORWAY. THE 1ST JUMPERS WERE UNAWARE OF ANYPROBLEM, BUT SAW THE ACFT ENTER A STEEP BANK, ROLL OVER & SPIRAL NOSE DOWN. 16 JUMPERS EXITED THE ACFT, BUT 3 HIT THE STABILIZER. THE ACFT CRASHED IN A NEAR VERTICAL DSCNT. INVESTIGATION REVEALED THE CG WAS BEHIND THE AFT LIMIT FOR TAKEOFF & THE JUMP. THE ELEVATOR TRIM ACTUATOR WAS FOUND IN A POSITION FOR FULL NOSE-UP TRIM.