Magic?
Master Craftsman.
As far as PFS play, no, specific armor/named armor is not able to be 'upgraded' or improved. It is what it is in the description.
For a home game it is up to the DM, so talk to them. Rhino armor was apparently made for a specific character concept and adding extra damage to other concepts on top of excellent armor bonus could be a slippery slope.
Generally speaking, it's difficult to determine the appropriate "value" of the non-standard abilities when upgrading many specific armors and weapons. Should the specific abilities be valued as fixed gp amounts or as "+x equivalents?" Some items are simpler to calculate than others, of course; however, there's enough variation to require the GM to consider each item separately.
About the simplest alteration for many specific armors or weapons would be to change the material. Unless the armor or weapon description already specifies a special material (breastplate of vanishing, celestial armor, bastard's sting, boulderhead mace, etc.) it only requires switching out the price of a normal masterwork item for the price of one made of a special material (for instance, plate armor of the deep made of mithral or a bloodthirst dagger made of blood crystal).
Short answer: ask your GM.
Full answer: ask your GM, since the resulting armour would be a "home-brewed" item.