If the host and dopant atoms are isoelectronic or with valence states different by +/-1, you can treat the effect of doping within virtual crystal approximation (VCA). For this, you need to generate a new pseudopotential representing the virtual atom resulting from the doping. For more details, please refer to this tutorial: https://www.tcm.phy.cam.ac.uk/castep/documentation/WebHelp/content/modules/castep/thcastepdisorder.htm

Thank you very much @Mohammad Saeed Bahramy