THE FLIGHT WAS A MULTI-ENGINE CHECK RIDE FOR THE PILOT/APPLICANT. BOTH PILOTS REPORTED THAT AS THEY WERE ON A SHORT FINAL APPROACH THE AIRCRAFT 'ABRUPTLY OSCILLATED (BUFFETED), PITCHED NOSE UP THEN NOSE DOWN...' THE NOSEWHEEL IMPACTED THE RUNWAY AND BROKE OFF, AND THE AIRCRAFT SKIDDED OFF THE LEFT SIDE OF THE RUNWAY. THE PILOTS STATED THAT THE AIRCRAFT HAD ACCUMULATED ABOUT 1/2' OF MIXED ICE DURING THEIR DESCENT THROUGH THE CLOUDS. THE AIRCRAFT HAD BEEN MODIFIED WITH A ROBERTSON STOL KIT IN JULY, 1976. THE AIRCRAFT IS NOT CERTIFIED OR EQUIPPED FOR FLIGHT IN ICING CONDITIONS.