To be honest although that can work for simple campaigns the more complex you make a mission the harder it becomes. The idea of turning
Birth of a Legend into a multiplayer mission without drastically cutting it down would give me the cold sweats and if it comes down cutting features out of the SP missions in order to make them multiplayer compatible I think you can guess what my response would be.
That said I'm not averse to making a cut-down multiplayer version of some of the missions whenever it is possible. If it's not I'd rather just make a new multiplayer coop campaign instead.