Cosmic ray showers can serve as important calibration sources for in-ice radio detectors aiming to detect the cosmic neutrino flux at the highest energies. However, if not well understood radio emissions from cosmic ray showers pose an essential background signal in the neutrino search. Thus, we have adapted the CoREAS air shower radio emission simulation code to simulate air shower radio signals for in-ice antennas. We present a novel upgrade CoREAS such that it takes into account curved ray paths caused by the exponential refractive index profiles of air and ice. This enables propagating signals from air to antennas located inside the ice sheets.