[Ternneowns]

It seems Maha Bharata had a line that described a conversation between Indra and Yudishtira (I think) mentioning the allowance of "mlecchas" to re-practice their original faith and some rites for that. You have to judge them by the gunas and slot them in the appropriate varna, though I think this is best left to Devas when they ever come here, because most of the explanations, interpretations and full texts have been lost due to time, invasions, interpretations for personal gains, etc.
But there is a rule and I am sure it exists. Also, I am sure it is very liberal just like rest of scriptural Hinduism is.