Taking into consideration the birth based practice of Varnashrama prevailing for a couple of millennia, kindly give a clear scriptural sanction/ permission from the ultimate— apaurusheya Vedas, Smrutis or Upanishads only— for converting non- Hindus to Sanatana Dharma ( excluding (ghar wapsi). Pl do not provide historical examples or opinions from saints, neo- vedantists/ universalists/reformers etc. They are more like exception.
I have checked all the other answers suggested to me. These are full of opinions and overwhelming number of them are not backed by any scriptures. Saying Shruthi is silent about it because back when Vedas were bestowed on us, there were no other non-hindus is a speculation. Secondly, if Vedas only dealt with what was the then prevailing situation, we are indirectly saying they are obsolete!! Secondly, the much touted Arya Samaj is mentioned as doing shuddhikaran conversions based on Devala Smriti but if that is so, this Devala Smriti deals with only reconversions or ghar wapsi and not conversions of non-Hindus. This is what I find-- K.M. Munshi in his work "Chakravarti Gurjaro translated into Glory that was Gurjaradesa says quote-
Devala, the author of the Smriti of the name, is placed between A.D. 800 and 900, when the fortunes of Islam in Sind, as stated before were on the decline. He also wrote his Smriti while in Sind. The movement represented by him appears to be largely responsible for the active campaign of reconversion from Islam, which led the Muslims to seek asylum in al-Mahfuzah, a fortress specially constructed for the purpose. Devala gives sanction to the practice of reclaiming mlechchhanita - a person converted by the mlechchhas. It deals with the problems of those who were kept as slaves by the mlechchhas and compelled to do unclean things, like killing cows, sweep the leavings of the food taken by the mlechchhas, taking flesh of asses, camels and pigs, and the forbidden food or drink. As regards women abducted or raped by the mlechchhas, the smriti shows a breadth of vision difficult to find in any Dharmasatra of later age.
So, this Devala Smriti written in 800 AD after the advent of all the three Abrahamic religions is the only text I could find and that too only for ghar wapsi which is not what I have asked about.
My question is not asking for opinions, historical examples or certificates given by converting bodies but the shastra pramaana that all of them should be finally relying on.