Hi Robin, I was wondering if you found the info you needed?
I've never had to incorporate an addendum so wasn't sure about this one, though I've seen them used... here's what answers.com says:
an addendum is an additional document not included in the main part of the contract which may contain additional terms, specifications, provisions, standard forms or other information. A contract addendum may also be called an appendix or a rider. Addendums are often used in standard form contracts to make changes or add specific detail. For example, an addendum might be added to a contract to change a date or add details as to delivery of goods or pricing. The Addendum should be referenced in the contract, or the contract should be referenced in the addendum, so that it's clear to which contract the addendum is modifying.
I had a hard time finding a sample that applied to a freelance contract or similar and only ultimately found one
for a buyer/seller relationship but I think you can easily update the language to apply to your situation - the format/body should give you a good idea of how it should be laid out.