sure can do

search the fortresseurope.xml file in your campaign folder, copy and rename it
open it with notepad and change the sections you want.
save it and you're done

The sections that I change the most are:

<params>section; daytimeonly, set to n means you have to fly missions at night (great if you fly bombers like the mosquito), duration (in days) defines the lenght of the campaign (o=endless)

<startDate>section; defines the start date of the campaign

<alliance>section; the invadeon controls which alliance can invade first

<TechTree>section, here you define which planes (stock and addon) are available and from which dates (to be realistic you better use the entered/left service dates from their own files). The names used are that from their .xdp files if I remember it correct. Keep in mind wich is the start date of your campaign (see above)

BTW; their is also a swap file that is available (sometimes included in the NEK reduction kit), that makes the English planes to show up as the enemy in axis stock campaigns. It is available at www.sim-outhouse.com or here at simviation.