At this time, there is no official sync service between Nike and Strava, however, there are a number of 3rd party solutions built by developers using the Strava API.
Please read the comments from the community below as many of the existing sync apps have been disabled due to changes with the Nike+ API.
Some sync services to consider:
RunGap for iOS - workout data manager app (VIP in-app purchase needed)
Sync Smashrun with Nike+, Then use tapiriik to sync Smashrun with Strava (http://www.nikeplusexodus.com/) If you have problems with tapiriik, you can export activities from Smashrun by tapping the download icon highlighted below. You can then upload the activities to Strava