This feels like a question better left for when scripted missions start showing up for even IA. Its all done in lua now, so there's the first major barrier for mapmakers. Once someone has a handle on all that though, it should definitely be possible. Some things may need to be tweaked just from the nature of MP & multiple people (might have to lock out joins after start) but yeah, doable.

You could make the SP missions into single MP-missions, but you could not string them in a campaign. Heck, atm you can not even stay in a game when it finishes :D

For Mechwarrior 4 Merc CO-OP Mod, they did something similar where after each mission you set up the next in a MP lobby as they were titled 'Urban Mission 1, 2, 3 ect' I just have some friends who don't share my nostalgia goggles that would only enjoy the campaign with someone who knows what they're doing.