DRG TKOF FM RWY 19, THE PLT TRANSMITTED 'WE HAVE A DOOR OPEN.' TWR PSNL CLRD HIM TO LND ON ANY RWY, BUT HE STATED AGAINTHAT A DOOR WAS OPEN. AGAIN, TWR PSNL CLRD HIM TO LND ON ANY RWY. NO OTR RADIO CALLS WERE RECD FM THE ACFT. SVRL PERSONSSAW THE ACFT AT LOW ALT BFR IT ENTERED A STEEP TURN TWD RWY 24. SUBSEQUENTLY, THE ACFT HIT A TREETOP, CRASHED WITH THE FLAPS RETRACTED & CAME TO REST APRX 300 YDS FM THE THE RWY THRESHOLD. THERE WAS EVIDENCE THAT THE UTILITY & LEFT CABIN DOORS WERE CLOSED DRG IMPACT. HOWEVER, THE RGT CABIN DOOR WAS FND WEDGED UNDER THE RGT WING WITH LITTLE DEFORMATION. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FND DRG THE INVESTIGATION. TESTS SHOWED THE FLT CHARACTERISTICS OF THE ACFT WERE UNCHGD WITH AN OPEN DOOR; HOWEVER, THE FLT MANUAL RECOMMENDED USING SHALLOW BANK TURNS, CAUTIONED ABOUT POSSIBLE BUFFETING & A SLIGHT INCREASE IN DRAG, & WARNED NOT TO ALLOW THE AIRSPEED TO DROP BELOW THE RECOMMENDED SPEED.