An extension of the Hermite Expansion Method for performance simulation of central receiver plants is presented. This extension allows simulation and parametric study for a solar central receiver plant based on Solar Concentration Off-Tower (SCOT, or Reflective Tower) design. The extension includes mapping the physical receiver aperture into a Virtual Receiver located near the field's aim point, and performing the Hermite Expansion calculation on the Virtual Receiver. The calculation of aperture intercept/spillage and additional losses due to the Tower Reflector's finite size are discussed. Validation of the extension by comparison to ray-tracing simulation is presented for single heliostats, a group of heliostats and a complete surround field. The results match closely, showing the validity of the method and of its implementation.