A rider can be written into basically any type of contract. The basic concept behind a rider is adding some type of information or clause to a contract that already exists and that has already been agreed upon by all parties. Just because a contract has been finalized and signed doesn't mean that changes cannot be made to it. As long as all involved parties agree on the changes, writing in the rider should not be an issue.
Agree on the terms of the changes being made to the contract. Discuss these changes with all parties who originally signed the contract and come up with something that works for everyone.
Write out the changes that you agreed upon. For example, you might be changing an employment contract to include additional vacation days. In this case, both the employer and employee who originally signed the employment contract should review the written changes.
Add the rider to the contract under the appropriate section, or add the rider at the end of the contract terms.
Ask that all parties involved with the contract sign the rider. Signatures should go underneath the rider to make it clear that these changes have been agreed upon by everyone involved.