am i able to add a new custom gun if i have ugx installed on my map?

I don't know why you wouldn't? Just the same as adding a weapon to your map normally except instead of modifying a version you create of _zombiemode_weapons.gsc you need to edit the one in the ugx_mod.iwd. Then include the weapon in a CSV. Also keep in mind you need to include the weapon in ugxm_user_settings.gsc as well for compatibility with our gamemodes.