Due to the differences in actions this isn't possible as far as I know, especially when it comes to converting Infopath into Nintex forms - the formats are so different that it isn't possible with any tool that I know of at this time.  For the most part you will find this to be a manual conversion.  The good news is that it is really easy (compared to OOTB) to get workflows up and running quickly.  Nintex Forms are nice and straightforward as well.  Let us know if you do come across anything nifty in your search.

Thanks,

Mike