Cosmic-ray (CR) air-showers impacting a high-altitude ice sheet impart a significant fraction of energy into the ice, forming a dense cascade below the ice surface. Above PeV energies, these secondary CR cascades are the target of the RET-CR experiment, and their associated radio-Askaryan emission makes them a background source in radio-neutrino detectors, such as ARA and RNO-G. This process can be simulated using Monte-Carlo techniques; however, these methods become computationally expensive at higher CR energies. In this work, we explore an alternative method to generate the in-ice continuation of a CR air-shower simulated with CORSIKA. We show that using the particle content at the air-ice interface in combination with standard air-shower parameterisations allows the resulting in-ice cascade to be obtained quickly and accurately.