Is there a way to turn a single mission into a multi-player?

tia

Yes. Open the mission you want to change in Full Mission Builder (assuming you know where it is saved).

Now you will need to choose which units/flights you want to be AI only. There will probably be other changes you need to make, like uncheck the option of making a certain plane the player plane, and also add seperate briefings. Then save it to the NET>COOP folder in missions.

I dont recommend this though, just learn to make your own.

An even easier way would be to use Quick Mission Tuner. First copy the single player missions into a folder under coop as danjama points out, then edit them in the fashion he mentions but with the QMT (a wonderful and easy to use tool).

Find QMT here:
http://www.mission4today.com/i...file=details&id=3451