The pilot received two weather briefings; both indicating icing conditions en route to Hays. In addition, both briefings advised of a SIGMET for reported moderate-to-severe icing in freezing rain below 4500 ft MSL. He telephoned both dispatch and the chief pilot; the consensus was that the weather had not developed as forecast and the adverse weather appeared to be east of Hays (1st landing location). While descending into Hays, the airplane accumulated ice rapidly. The pilot did not see the field at MDA and executed a missed approach. He was subsequently unable to climb out of icing conditions, and the airplane settled to the ground with full power. The airplane was not equipped for flight into known icing conditions. It did incorporate STC SA2573CE which installs synthetic rudder strips on the wing/tail leading edges. The anti-icing 'shoes' are sprayed with glycol to help prevent ice build-up if icing conditions are encountered in flight. The pilot stated that the owner represented the STC as meeting FAR 135.227 for flight into icing conditions.