It’s MUCH easier than that:
Go to the panel for Domains -> Manage Domains. You’ll get a listing of that bunch of domains you have. Click Edit in the Web Hosting column for that domain.
You’ll get the light-blue Fully Hosted box with the current info. Part of the info is the FTP User, which is a dropdown menu. Go into that and pick Create New User, and then enter the new username right below that.
And Check the “Copy domain’s files to new user” box.
And that’s it! As a precaution, back up the files to your home computer first.