The work reported herein focuses on the extension of a well-established computer program to include the interaction of a nonlinear energy sink (NES) with an aeroelastic structure. We consider an initially linear structure in transonic flow and seek a realistic representation of the air loads acting upon it. For aerodynamic computations in this seed range, we have adapted the CAPTSDv computer program. It is possible to use almost all of the machinery of this program, including the modal representation of the linear primary substructure, while incorporating the equations of motion of the nonlinear attachment and replacing the time integration algorithm with one suited to the resulting state equations. A simple primary structure, a rigid airfoil with its heave and pitch motions restrained by linear springs, was chosen for the first application of this modified code.