I looked into this a while back (nop 1.8 with sitefinity 3.7). It was going to be a lot of work and even though the older version of sitefinity was more open, the two really didn't play well together in my opinion.
The new version of sitefinity is now full commercial and it is still based on web forms. Nop 2.0 is MVC.
If your looking to integrate a CMS with nop I would suggest something like N2 or if you are more courageous umbraco. Either way, it is not a trivial exercise.