There is nothing particularly objectionable about modifying items you already own for re-sale. By adding your own materials, intellectual property (the design), and not claiming it was made by the original manufacturer you should be within the limits of the law.
Christopher R. Minelli
Minelli Law Offices, LLC
NOTE: This answer is not intended to be legal advice and should not be construed in that way. This answer does not create an attorney-client relationship and no such relationship may be created absent a signed retainer agreement. The author is licensed in Illinois only, and his answer is for educational purposes alone.