In my opinion there are two ways to do this. First, a button to convert the weapon into the mods it's made of. The negative side of it, you lose a weapon, because you can't combine mods into 2 one functional weapon yet (I don't know how difficult this would be). This shouldn't be that hard but I think it's not possible yet. Second some sort of "unfinished" weapon state. Imagine a "none" option like the gun muzzle for every modding slot exept the receiver make it the important key part with the weapon id . If any vital slot (like grip, or magazin) is missing, the weapon is "unfinished" and can't be equiped. I have know idea if, or how you can make something like this work, but I suppose it's not possibel until the G.E.C.K release. Some mod I found to take the good mods without having to craft the standarts to replace them. It removes the "cost" to build them. Maybe it's good for now http://www.nexusmods.com/fallout4/mods/2073/?