Yep, you sure can add instruments to a cockpit.
Have a look at the wiki page
Howto:Adding instruments to an aircraft and the topic
Adding instruments to a cockpit.
The biggest hurdle, at least if you have not taken a look at the file structure of the aircraft and/or inside their files before, is figuring out which files to look for and where to look in them. You should look for xml files. (Keep in mind also that that organisation is up to the individual aircraft developers, so each aircraft can expected to be slightly different in that regard).
A tip I use a lot is to use slightly changed coordinates of a nearby instrument and have a peek in fgrun's (do they use that on Linux as well) aircraft preview to check the instrument placement.