I would guess that, one of the reasons there are some reins on crafting a this early stage is that the crafting system in Skyrim was far too open-ended for it to be balanced with the rest of the game design. Sure, it was way cool that you could make lots of stuff from scratch, and then as you leveled up make virtually anything . . . but because of so many other aspects of the game, the consequence of going on crafting sprees was that it could either make your character Godlike or underpowered, depending.
I have already found myself using the crafting system to level up in a somewhat "gamey" manner, but thankfully the maths they have set up are fairly tough when it comes to this, and this combined with the relatively limited materials for crafting means that it is not nearly as vulnerable to abuse as crafting was in Skyrim.
This in mind, I can see why they have erred (at least initially) on the side of limiting what can be crafted. There are certain items in the game that are pretty readily available and of otherwise fairly low value (long johns, raider leathers, road leathers, basic civilian garments) and if all that stuff were "craftable" it would make it far too easy (and too tempting for some of us) to abuse and level up incessantly.
Possible solutions:
1. Crafting of anything that is not presently craftable nets zero (else for say a 12 INT character) 1XP. That would nerf its vulnerability to abuse.
2. Make the monetary value of such combined or enhanced stuff relatively slightly increased by modding.
3. Make the material costs reasonably high. Adhesive seems to be one of the more scarce commodities, so for example: To combine a helmet with a +1 PER cap materials might be something like: A. the two items; B. 2 adhesive; C. 2 Leather; D. 2 cloth; E. 1 steel (rivets, buckles, etc.); F. one plastic.
4. Make such combined crafting available only with one or two perks in the appropriate perk paths (e.g., Armorer 1 and Scrapper 1 for armor versus Gun Nut 1 and Scrapper 1 for ranged weapons and/or Blacksmith 1 and Scrapper 1 for melees.
5. Always cause some mild degradation of the primary items in the combination. For example adding a +1 PER fabric cap to a helmet might cause its ballistic or energy protection value to drop by 1 or 2%.
I haven't thought through if these ideas are really appropriate to the weapons, but for the armor this sort of thing would be neat.