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 &