Thank you for providing your state name, Bigfish.
The sculpture you registered is considered your "underlying work" for the purposes of registering your "derivative" works. A derivative work is any work that is built on or has as its base a preexisting work.
Often when creating a work based on an existing work, permission is required from the author of the original work before the derivative can be made without infringing on the rights of the first author. Because you are making derivatives from your own work, this permission is, of course, not required. You are allowed as the artist of the first work to create derivatives of that work. Making derivatives is one of the bundle of exclusive rights given copyright holders.
The additions to the original work must be creative and original enough to be eligible for copyright protection because, when you file for registration on your new sculptures, you will be registering only the additional material. You will, however, need to identify the original work that you based your new sculptures on. You can identify the underlying work through its copyright registration number and you will be providing a brief description of the original material added to this underlying work.
Using your example, your original copyright covers your first sculpture - the broomstick with the dustpan. This is the underlying work. The other sculptures you create based on the broomstick and dustpan work can be registered and protection can (potentially) be afforded. In other words, the mop addition is what will be protected. You can only claim what is new. The original material will be disclaimed.
For more information, you can visit the Copyright Office and check out Section 101, Definitions, for the definition of "Derivatives." You can also check out Section 103, Subject matter of Copyright, for additional information on derivatives. And you can check out Circular 14, Copyright Registration of Derivatives. Here is a link to the Copyright Office: http://www.copyright.gov
and here is a link to Copyright Circulars: http://copyright.gov/circs/