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@FredKay Hi there! They don't sync to each other directly, but you may want to sync your own data through a third-party app. This won't ensure that it will export previous data, it may start to sync between sites once you connect them.
For example, SyncMetrics. This app syncs Nike+ activities with Fitbit, including weight. Besides Nike+ it also help you synchronize and analyze fitness and health data from Cycling Analytics, FatSecret, Garmin, iHealth, MapMyFitness, Nike+, RunKeeper, Strava and Withings. Perhaps you can do it backwards with this option. Let me know if you have another question, and happy stepping! 
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