You could do it by extracting them from... er, whatever VP the missions are in.

I'd check but my computer is doing something... odd, and I can only browse the Internet, not access files (long story).
But, for the love of all that is good, do NOT CHANGE ANYTHING WITHIN THE MAIN VPS! Extracting a mission does not count as changing... just don't put the VP together with the new mission in place. Bad idea.
VPView is a good program for extracting. I'd give you a download link... but descent-network is down, and so, therefore, is the only link I know. If there is a mirror, could someone else post it?
Anyway, you'd extract the mission, mess around in FRED, then you could put the mission into Data/missions. I believe the campaign would load it instead of the original mission. When you're done, just remove the mission from Data, and everything's back to the way it was.