[kaybe]

So what would happen if, say, an Australian or Haitian converted to Hinduism? Or is that not possible?

[devdas]

Traditionally, you couldn't convert to Hinduism (Hinduism is literlly the set of religions followed by people in South Asia). In modern times, you can convert to Hinduism.

You would end up without a caste, but with higher or lower social status depending on your original country.