When in the editor go
File -> Open Song Folder
There you can find the mp3 the map used. Copy that mp3 to your desktop and drop it in the editor.
Enter Metadata, place one note and save it, then go File -> Open Song Folder there as well. Then leave the editor(!)
Open the .osu-file of your difficulty in the old folder within Notepad. Copy everything below the [Difficulty]-line.
Open the .osu-file of your fresh difficulty in the new folder within Notepad. Mark everything below the [Difficulty]-line and replace it with the stuff you copied before.
Save the file in notepad and close notepad. Restart osu!. Now your newly created map should contain your difficulty.
Make sure you credit the original map within your submission-message.