Oh, good topic raised Adai!
From what I know, there isn't such a thing as converting to an Hindu.
You can be an Hindu by just following some of the teachings & duties of an Hindu as prescribed by our scriptures.
Yet again, those who are born as Hindus, how many of them atleast know the basics of their religion?!!
Hinduism says that God is one. It clearly demonstrates the many paths to God as well!
As such, it does not see the different paths to the one God(the different religions) as something completely beyond it (its principles & teachings etc).
In fact, it can be said that by being a true Hindu, you tend to appreciate all religions.
You can be a Hindu, n still pray to Mother Mary, Jesus Christ, visit mosques, temples etc and you are still an Hindu, in no way less..
Our tradition has sayings such as, "Yem mathamum Samatham"
- Hinduism is a way of life. It is something entirely dependent on the individual.