I think that if you have the Game of the Year version of FS2, the Templar Campaign (the one depicting the downfall of the HOL years before the Main Campaign) is made available for single play. However, if you didn't purchase that version I think you could always just choose a LAN connection and play the campaign by yourself. I know I've found a way to play Templar singleplay by bypassing the online stuff, and I'm pretty sure you'll find out how to do it as well. If all else fails, you could always extract the campaign missions from the .vp file, edit them in FRED2 so they're all single player, and then play them like normal user-made/single missions.