Games : How to add an enchantment onto held item in Minecraft with command blocks

Ok so I spent a lot of time researching command blocks but I still cannot find a solution to what I want in my game. I want to throw hit a button to enchant a piece of armor in my hand with unbreaking 5 only if it has unbreaking 3 already on it + 30 lvls. I thought I found a solution, by using a /testfor for a certain enchantment on a certain item ID range, taking away 30 lvls, and then enchanting the item in-hand with /enchant but it turns out you cannot use /enchant to make higher than default item lvls. You can only do that with the /give command. I do not know how I am supposed to use the /give command because the item given has to have the same previous enchants as the item in my hand, but also with unbreaking 5 only if it had unbreaking 3 on it. There must be a solution! Please help!