Converting a single ownership into joint ownership

Hi, 

I am sorry if i posted this question to the wrong thread.

My query is that my sister got a flat in a state level housing scheme. The application was on her name. We both want to make the payment for this flat. Now we want to convert her ownership into joint ownership (my sister and myself).   Is it possible at all?

In any case, does the answer changes if the possession is yet to happen. What are the other options so that we both can have equally rights on the flat.

Thanks a lot!