Yes you are allowed to add an addendum to a “Standard” Real Estate Contract. We often help clients add one addendum or more.
Associations that generate standard forms generally have standard addenda to accompany those forms. Sometimes you can use those. In other cases, when either there is no access to such standard addenda, or there is none that fits your situation or purpose, your attorney can put together custom addenda to present to both sides for signature.
Buyers and Sellers should recognize that the protections offered in standard form contracts and addenda might not match up with the protections they would want in the deal. See our post on standard form contracts here.
Banks, relocation companies, institutional housing investors and other parties connected to a deal all have their own standard addenda that they may require to be attached and incorporated into any residential sale they are apart of.
Both Buyer and Seller should be aware that the right to attach such modifications to the contract is not exclusive to big parties, such as lenders and title companies. Buyers and Sellers also have this right and should feel free to exercise it upon the advice of their attorney, as appropriate to the circumstances of the deal.