Dear Paolo,

You may have heard of Marco Polo's description of alchemists in India and also of Al Biruni describing rasāyana. 

Both accounts are featured in David White's The Alchemical Body. There is also the story of Siddha Iramatevar/ Yakoppu visiting Mecca. 

I don't know of any journeys by European or Arabic alchemists to the Indian subcontinent. However, Fabrizio Speziale may know something about that: He has been working on Persian sources on alchemy and iatrochemistry and these may feature accounts of visiting or at least corresponding alchemists. 

Very best wishes,

Dagmar