Add NBT data to an existing item

I need a way to dynamically add NBT data to an existing item, what I'm trying to do is make a command that changes a player head to use a different players skin based on what a player types into a command, I have the command part working but I can't find out how to directly add the NBT data or component to the player skull, since all I can find through the blocks is the "add custom data" blocks which don't do what I want.
 

I'm using MCreator 2025.3 and the mod if for Minecraft Version 1.21.1 (Neoforge)