Absolutely it is allowed however in some ways one can say everyone is Hindu. Hindu is one who performs Karm (actions) in the name of GOD. In Sanatan Dharm the whole world is one large family. So if one performs his actions in life in a peaceful way and pays respect to Sri Bhagwan then he is a Hindu. If one is a Hindu Muslim Christian Sikh etc…. and not following its teaching then he is fooling only himself. So conversion doesn’t mean anything. In many ways conversion is about dividing people into groups and making it a popularity contest. This is not a path to GOD.