It looks like there is not currently any tool to do this in an automated fashion.  

If you go to the list settings and select manage forms you can reset the form back to default then use Nintex to create it.  Works well.

Hi Brarathi, this has been coming up a lot more. When you say convert, what were you looking for with this? InfoPath and Nintex forms somewhat coexist in SharePoint. You can also dissassoccitate an InfoPath form and use Nintex quickly.  

If you want to try and move the form to Nintex, we are working to help users do that. It's not easy and a lot of functionality doesn't correlate as easily as one would think. Happy to help if you have any questions.