Do you mean you want to move the item you made into a different .user file? Because if so, the quickest way to do that would be to open each file in a text editor and just cut-paste the XML block.
You could also open the file you want the item to live in, copy the item, save it under a new ID, then open the old file and delete the original, and finally go back to the new file and change the item's ID back to what it was. That last step is optional if you don't care what the actual item ID is and you don't have the item on any existing characters.
Before you do this, copy the file to a backup folder, so if anything goes wrong, you're not stuck with un-fixable errors.
If you're not used to editing XML code, or if you're squeamish about editing it, then you can copy the file to a backup folder (this should ALWAYS be the first step), then make a copy of the file within the same folder (with a different name, obviously).
Then open the new file in the editor, and delete everything except the new stuff you entered.
Then open the old file in the editor, and delete the new stuff.
OR ...
Copy the file to a backup folder.
Open a new file in the editor.
Create a new item, but create it from a copy. When the list pops up, search for and select the item you wanted to save in the new file, give it a name and a unique ID. Save the item. Save the new file.
Then open the old file and remove the new item.
