Can you swap weapon mods from one gun to another?

I have a decked out 10MM and a decked out laser rifle, but both of them are missing a mod I have on another gun? I can't figure it out for the life of me

Also, when looting an item has a + beside it, what does that mean

I would love an answer to the swaping as for the plus sign i think it means it does more damage or protects more then what you currently have on.

Yeah, if you try to change mods from the inventory it says that you can only do it at a crafting station but if you go to the crafting station it just creates a new mod rather than using an existing mod.