Yes you can
I'm not at home but give it a try to remember it

Open your C:\Documents and Settings\YourUserName\Application Data\Microsoft\ folder and you will find a file that is called something like fortress europa.xml

first make a backupcopy of it

then open it with notepad and scroll down till you find the name of aircrafts and the entered service dates etc.

you just can add extra planes with their names from the aircraft folder, just be sure that the start date of the campaign (somewhere in the same file) is passed the entered service date of the aircraft you want to fly

instead of making a backupcopy you can also add a new campaign with a new name and leave the stockcampaign unchanged, otherwise you wan't be able to continue the stock campaigns you have already started

If you need more explanation you should wait till monday to hear it from me, back at home I temporary have troubles getting on the internet