@Foxtrot I use Photoshop, but GIMP will work just as well, basically any image editor with DDS capability will work.

do i need any kind of programs for that ??(ofcourse i do, but what programs are these???)

@Foxtrot probably, replacing an existing uniform texture would be the simplest - find the uniform textures (should be in Data/HardLife.zip/MEDIA/ACTORS , the invertory model tex. are in ITEMS) , edit them and place in your mod dir. with the appropriate dir and name. The models in ACTORS dir seem all to be DDS images renamed to TGA, unsure if you need to export it as DDS as well.
I haven't added a new item to the game, but i'd start by tracking down where the other uniform definitions are stored and copying those definitions to use my data. Haven't noticed where those are stored so cant help you with that.

is there a way to make an outfit mod of WWII Paratrooper, 80s Russian and american Uniform and a Tuxedo Suit outfit ???