Any Mlechcha(non Hindu) can become a Shudra(hindu) Indo European Scythians or Sakas(Kushans are a sub tribe of Saka) ,Greeks,Persians and Turkic Huns,Jhanjhars all became shudras and OBCs in the past.
To become Brahmin he can perform penance and austerities(Tapasya) and get Brahma the creator's acknowledgement
