Hi, is it possible to just reskin a weapon with custom texture ? (mp40) And add it to our map ?

Yes, you'd have to use Tom-BMX's IWI & DDS converter to export the MP40 image to .DDS, then load it into Photoshop or Paint.net (you'll need a plugin to load .DDS images, just google search for it). You can edit the image in there and save it & convert it in Asset Manager, and include it in your mod making sure the image has the same name etc.

Thanks !