Have you tried setting the move effect byte to 282? You can do it with the latest version of PK3DS. If you've already tried that and it hasn't worked, it could be that the Psyshock effect is hard coded somewhere else (like Thousand Arrows and Scald's defrosting effect).
If you want to hex edit the moves, the best approach is to first unpack the move garc (a/2/1/2) using garctool, which will create a folder containing each move in an individual file. They'll be numbered according to this list:
http://bulbapedia.bulbagarden.net/wiki/List_of_moves. From there you can open the move you want with a hex editor and change whatever you want.