As long as it is not listed on any exchange which it seems like it at the moment, there is absolutely no way you can convert your rinkeby to ETH and you will just have to be patient and keep monitoring the team until you get a hold of the news that it has been listed. 

Moreover like some people have rightly said, this is just a test net; so there is no need bothering yourself, as you would most definitely not get anything from it. What I would say is not to sweat too much on getting anything out of this.