There were a couple of conversions for X-Wing and Tie Fighter to be played in Alliance. I'm not sure if these work anymore though with the upgraded Alliance Craft pack or with Gog. Just make a backup of your copy first if you want to save your pilot.

http://www.maxgames.it/xwc/

The old games weren't exactly moddable the only time I tried a mission pack with X-wing hasn't gone well.
You can configure the DOS games to use a better soundbank:
https://www.gog.com/forum/star_wars_xwing_and_tie_fighter_series/get_dramatically_better_midi_music
(I strongly recommend Timbres of Heaven)

And the windows games have wrappers that allow higher resolution gameplay (though not widescreen, only stretch if you want), a version of which is already included in the GOG releases IIRC.

X-wing alliance is the most moddable, the most famous one being this mod for the graphics:
http://www.xwaupgrade.com/

Thanks guys thats very helpful !
All the best

You can find a lot of mods and missions for XvT and XWA on Jefe Picaro's website:
http://xvtedicion.es/ingles/index.htm