I would imagine that all you'd have to do is edit the stock missions aircraft choices using M$ Mission Editor which you can download from their website.
Of course I'm an FD specialist not a mission designer... failing that there are plenty of really good missions available for download that should fill your hearts every desire for destruction and mayhem

Ozzy

Ooops you might be right there chap, I haven't played CFS3 for ages!!! The braincell needs winding-up, I'll just go and shove a key in my ear

In the CFS3\campaigns folder is a file called fortress europa.xml.
Open this one in a text editor or a special xml editor if you have one.
Locate the techtree section and add any plane you wish there using the same format as shown.
Use the name of the aircraft's main m3d file as found in the aircraft folder and add a service entry.
Save, start a new campaign.

BTW, the Mossie doesn't show up in the fighter career as it's a tactical bomber in CFS3.
You can only select it in a bomber career if or just alter the aircraft's xdp in the first line where it sais "tactical_bomber".
Change this to "fighter_bomber", save, delete the plane's bdp file (will be created new automatically) and a start a new campaign as mossie fighter pilot
note this tweak will give you missmatches when you fly online.

I finally got round to trying it and it works a treat. Now I've got the MkVI Mossie, strafing and bombing factories, V1 sites etc are a whole lot more fun.

Thanks for your help.