FYI - haven't tested it yet, but to convert the planes to coop seats, you could simply open the .mission files using wordpad, do a "replace" on all instances of "CoopStart = 0" with "CoopStart = 1" and save it, including the .list and .eng file types of the same filename (example Dogfight5SpadXIIIv5AlbDVa.mission) in your Multiplayer/cooperative directory.
At least until Laser includes the coop checkbox on the next version

Edit: whoops, I think you need to change the mission type as well. Within "options", make sure you set: MissionType = 1; (versus the MissionType = 103; it is now)