"supplemental agreement" is correct and usable in written English.
You can use it when referring to an additional written document supplementing a contract or to an amendment to a contract. For example, "The parties entered into a supplemental agreement to modify certain provisions of the original agreement."
As a result, Season Smart had entered into a supplemental agreement to advance $700 million.
Trump wanted the Europeans to cobble together a supplemental agreement that would impose further limits on Iran's ballistic missile program and get rid of the deal's sunset clauses — expiration dates on some of the restrictions on the country's nuclear program.
Three days after the preferred stock was allotted and without the benefit of professional advice, a "supplemental agreement" was signed by Olympus officers that gave AXAM power of veto over "any important decision in the Gyrus business".
Nothing in this section shall be construed to preclude the adoption of appropriate security precautions to be determined by supplemental agreement between FAO and the Government.
In 1992, candidate Bill Clinton, running as a centrist Democrat, endorsed the controversial NAFTA agreement, but insisted on the addition of "supplemental" agreements on labor and environmental protection which were negotiated once he took office.
Supplemental Agreements and Settlement of Disputes.
The Government and FAO may enter into such supplemental agreements as may be necessary within the scope of this Agreement.
SECTION 28 The Government and FAO may enter into such supplemental agreements as may be necessary within the scope of this Agreement.
Things changed in July 2011, when Greece signed a supplemental loan agreement with international lenders to ward off financial collapse.
Klich said he expected the Polish Sejm and Senate to ratify the supplemental SOFA agreement by the end of February or early March, and hoped President Kaczynski would quickly sign.
This work is being completed through a US Department of Energy (DOE) Cooperative Agreement with supplemental funding from ADA, EPRI, and several power industry partners.
