In theory - it sounds like it should be doable. If HCl can plug his multiplayer code into the sim missions, he could undoubtedly plug it into a campaign mission, too.
In practice, however, given that HCl has been almost completely inactive for the past year or two (due to lack of time and other factors), it seems pretty much impossible. I don't think HCl has made the source code for the MP patch available, so it would be hard to get someone else to continue. In addition, a campaign mission is somewhat more complex than a sim mission, because there are so many computer-controlled ships around. That means there's a whole bunch of control sync issues that HCl has probably never touched - you might end up with the AI ships doing different things on each of the computers involved in the MP game.
It's probably not impossible, though. You'd just need to get in touch with HCl, and either get him to carry on with the project, or get the source code out of him, and then get some other programmer to help out. It would be a big technical challenge, but it might be doable.
