In that case, if you don't care much about balancing, it should be as easy and quick as karajorma says

Won't even need to convert the missions actually, you can just use the FS Port missions as a base.
First, extract all the missions and the campaign file with vpview. Then simply go through each mission, set the mission to multi+coop in mission specs, make sure it supports as many players as you want, and save it. Then go to the campaign editor, open up the FS1 campaign, set it to multi+coop, and save it. Don't change any mission names or it'll mess up the campaign. I think that should be about it.
You'll have a ridiculously easy time with it without rebalancing, but I'm sure it'll be good fun regardless
