no, you will not destroy any of the properties of bronze armor if you add a resistance.
Bronze armor has damage immunity, resistances add damage resistance, and as such they will coexist.
What you're probably thinking of VGN is using an elemental rod on a bronze weapon. The damage bonus from bronze will not stack with the damage bonus from the rod, since they are the same type of property.
However, do not add an elemental resistance to anything that already has damage resistance (i.e. johans cloak, platinum clothing, or dire tiger armor.)