Note, I do no own CFS and cannot ensure that I'm right Though I think it should be possible. There realy isn't any point in having a mission with no ground defence. I think this would be possible but you had better talk tio Grumpy or Dakota about it. 

If you just want a tower that fires a machine-gun, then you can do ya own on the Mission Builder if you have CFS2. You put in a Control Tower American and put an Antiaircraft Gun .50 cal US on the tower. You can't see the gun in the tower, but the tower virtually 'fires' bullets. If you mean something in CFS1 or FS, then sorry I can't help. But if you do my thing on CFS2 you can turn it into permenant destroyable scenery by using Martin Wright's GSL tools. Go to his site and you need to go to some developer's page and ask for the GSL tools by e-mail.