Loving the game so far, but can't figure out how to swap mods from one gun to another (if I can?)

I have a decked out 10MM pistol, only thing it's missing is extendo clip. I just picked up a 10mm with the clip is there anyway to swap it to my decked out 10mm

And can I do this with other guns?

Totally separate question: when looting sometimes item has a + sign beside it... What does that mean?

From what I can tell the modifications are not olug and play so you need the required materials to swap back to another mod in that slot.

Doesn't look like there is a way to swap mods, which kinda svcks. The + means that it isn't just a base weapon as far as I know?