Is it possible for me to add markings to a downloaded and installed skin? If it would be how would I go about it?
You could just edit the skin and add the markings yourself
In the actual game there are already quite a good number of Squadron emblems and
many other nose-art for aircraft. Serial/Call Numbers and such are also available.
Anything else... you will have to create on your own skin, or add the design using
GIMP or photoshop into a skin-or-Void file
My suggestion... learn some aircraft skinning. It