i've personally never made a multiplayer conversion to singleplayer.
Do you mean what changes you have to make in the mission files? (actors.bin, scene2.bin, scripts)
Just have a look what changes been made of already converted missions, like Africa1(Airfield) e.g
Singleplayer missions can be downloaded on our Google Drive:
Hidden & Dangerous >
HD2 >
Custom Maps >
Singleplayer
If you want to create a new mission without overwritting an existing one, would mean to edit
Gamedata00.gdt
resp. Gamedata01.gdt
Never tried, so don't know if and how it works