AN F27 CREW; UNABLE TO MAKE A HIGH SPD TURNOFF AT RST; MADE A 180 DEG TURN ON THE RWY.

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DURING THE COMPLETION OF A SCHEDULED EVENING FLT; THE ACFT WAS LANDED ON RWY 31 AT RST ARPT; MN. THE PF; ALSO A CAPT FOR THE AIRLINE; PROCEEDED TO SLOW THE AIRPLANE IN ORDER TO MAKE ONE OF THE HIGH SPD TXWYS. THE ACFT SLOWED; BUT CONTINUED TO ROLL PAST THE HIGH SPD TXWY BY A FEW FT -- PERHAPS AS MUCH AS 10 FT. THIS REQUIRED THE ACFT TO BE STOPPED ON THE RWY AND TURNED VERY SHARPLY TO EXIT THE RWY ON THE PLANNED TXWY. THE PF ELECTED TO PROCEED WITH THIS AND INITIATED THE SHARP TURN ON THE RWY AFTER HAVING COME TO A STOP. I FELT THIS TO BE AN IRREGULAR MANEUVER; BUT CONSIDERED IT TO BE LEGAL AND WITHIN THE ACFT'S CAPABILITIES. THE PF; HAVING MADE THE TURN TOWARD THE TXWY; THEN FELT THE TURNOFF COULD NOT BE MADE AND PROCEEDED TO CONTINUE THE TURN ON THE RWY UNTIL THE ACFT WAS FACING 180 DEGS TO THE LNDG DIRECTION. I THEN INFORMED MY CO-CAPT THAT WE DID NOT HAVE A CLRNC FOR A '180 DEG TURN' ON THE RWY AND THAT WE COULD STILL MAKE THE TURNOFF ON THE TXWY. THE PF DISREGARDED THESE REMARKS AND PROCEEDED TO BACK-TAXI ON RWY 31 TO ANOTHER TXWY EXIT. AT THIS POINT THE TWR INFORMED US OF OUR NEED FOR A CLRNC BEFORE A 180 DEG TURN COULD BE INITIATED ON THE RWY. AS THE CAPT LISTED ON THE FLT RELEASE AND FLT PLAN; I SHOULD HAVE SUGGESTED CONTINUING ON TO THE NEXT TXWY; EXITING THE RWY BEYOND THE ORIGINAL HIGH SPD TXWY. THE PF; CAPT'S EAGERNESS TO EXIT THE RWY WAS THE INITIAL CAUSE; BUT MY LATENESS IN OFFERING AN ALTERNATIVE WAS A DEFINITE CONTRIBUTING FACTOR.