Spawn yourself as blufor and spawn empty NATO MK6 in editor, disassemble it then try to assemble it. No luck, right?
Now use my script which will force assemble mortar based on your soldier type. http://killzonekid.com/arma-scripting-tutorials-mk6-mortar-modification-script/
Assemble mortar, it is infact opfor mortar! To make sure:
hint typeof vehicle player;