Do you have to replace an in-game weapon to add a whole new one?

Hey there, i'm hoping you can help me out and maybe point me in the right direction?

I want to add a new item into the players inventory, with its own unique icon in the weapon selection menu. However, I don't want to replace a pre-existing weapon. Is this possible?

If not, does this mean that every mod created for GTA:V will have to replace something else?

Thank you in advance! 

(P.S Any advice/tutorials/links you have in regards to creating/modifying weapons is much appreciated!)