Hello Bob,
The product representation is an internal value used by the E-commerce module to determine certain behaviours during checkout. This value is not extendible, but even if was possible, you would need to have access to all of the E-commerce module code, and extend the logic anywhere where this the value of this field is tested. The best approach is to define a custom field and make your code use it instead. This can be done in:
Development -> Modules -> SKU -> Fields