No specific rules exist for crafting an already magical item. The possibility for adapting or crafting in such a way is left to the discretion of the DM for your campaign.
The rules in the DMG, pages 128 and 129, establish some minimum requirements for cost, time and effort beyond "talk to the DM" on magical item creation:
- Character must be a spell caster.
- The process can be sped up by getting multiple characters to contribute/help out
- Creation of any magical item requires the character(s) to spend time and money, and meet level requirements. (Per Table on p. 129 of DMG, excerpted below)
- The level of effort required is 25gp per character-day invested, plus the time, and whatever else the DM calls for.
The table below, from the DMG, covers creation of magic items, either as already shown in the DMG (potion of healing, +1 word, etc) or whatever you come up with as innovative and new. This should give you an idea of the cost and effort of pursuing an in game creation, from scratch or otherwise, as you work with your DM to arrive at a crafting that fits the campaign and the power/scarcity of the item.
Rarity cost level min
Common 100 gp 3rd
Uncommon 500 gp 3rd
Rare 5000 gp 6th
Very rare 50,000 gp 11th
Legendary 500,000 gp 17th
Modifying an already existing item into another form, if a Magic Item Formula (see p. 141 of the DMG) is provided by your DM, will have the how figured out. You would expect the cost to be similar to what is listed for the rarity of the item.
Bottom line: consult with your DM on how it will be handled, if at all.