Well, I disagree with you on that. A non-Hindu is free to follow Hindu beliefs eg. ISKCON devotees but that does not make them a Hindu. With Christianity and Islam, when you convert/revert you actually become a Christian/Muslim.
define a Hindu first.
you can very much take a non-Hindu to a mandir, where the head priest will do a 'Shrudikaran' on him/her, then take you into the religion, then you become 'religiously' a Hindu. you then change your religion officially, on your birth-certificate et al, and you then become 'officially' a Hindu too.