It's Possible, but it's up to a DM to homebrew
The DMG states in the Chapter 9 Section on Creating Magic Items:
The magic items in chapter 7, "Treasure," section are but a few of the magic treasures that characters can discover during their adventures. If your players are seasoned veterans and you want to surprise them, you can either modify an existing item or come up with something new.
While the existing list found in Chapter 7 of the DMG is not exhaustive, it is an actual list of items that are immediately available. As with nearly everything in D&D 5e, the DM can modify or create pretty much anything - but that doesn't make those modifications/creations any less 'homebrew.'. .
This is very similar to the language in the PHB regarding Equipment (Chapter 5):
table collects the most commonly available types
The types listed for Armor, Weapons, Magic Items, etc. are actual lists and not just examples - they are just not exhaustive lists.
If you are looking for an existing set of Adamantium Armor +2, you aren't going to find it in the books. It 100% requires the DM to create it and determine it's rarity and cost. They can refer to the guidelines for support in doing that, but the item isn't an existing item.
There are no existing guidelines in any rules book about upgrading a magic item. If and how this would be done would be a homebrew by your DM.