I'd like it only if it totally degraded the other features of the weapon, such as sawing the barrel off a .50 cal and making it a very slow reload pistol but was incredibly powerful at short ranges and useless at long range.
Otherwise it would just be "recipes" for uber-powerful wepons. That would be cool too, don't get me wrong

But the fastest thing to kill a game is being all-powerful - there's no fun if there's no threat.
What I mean is if you could modify a grenade rifle to shoot empty soda bottles, it should only be a shotgun substitute, not a long range kiling machine. But it suits your play, so it kicks.
If you could shoot vodka and whiskey bottles and it sets the target on fire when it hits. The cost of ammo would still be cheaper than some guns, but so worth it.
Duct taping three dirty water bottles to your hunting shotgun turns it into a slug-only gun that is accurate and more powerful than most rifles but not as long range, but way better than birdshot at close range and negates armor at close range, but adds 3 pounds.
Using a pilot light and embalming fluid to turn a BB gun into a poison dart gun that could kill anything short of a supermutant or deathclaw with a head shot or just disable a limb. But you get addicted if you use it too often
If you could optimize a particular gun for how you play, that would be nice. A single shot 12 gauge that was crippling at close range - requiring a single and a hunting gun, plus obligatory duct tape and wonder glue and a hotplate
A .308 service rifle that required all the service rifle upgrades first, then you add in a hunting rifle, using a welding gun or paying a Boomer or Gun Runner.
A plasma laser sniper rifle that used an energy cell and a mf cell that you could only get done by the Van Graffs if you didn't kill them. And killed someone for them every time you needed one.