IF it has 5 properties, then all you can do is change the values of the existing properties, on armor. Weapons and Jewelry have some properties that replace related properties, but Armor doesn't have that issue (Jewelry has 5 skill groups, where one skill in a group will overwrite another; On weapons, one slayer type will overwrite another, as do Hit (spell) types as a group, and Hit (area damage) types)
So, if you found something as loot that had 15/15/15/15/15 resists, it means that it has 400% intensity already, and that you could raise the total resists another 7 points, as loot items aren't exceptional, and have an imbue cap of 450% (remember to powder it first).
Then, if you wanted more resists, you could get one of the tools from UOGamecodes and enhance it with whatever metal floats your boat.