Hi! I recently gave my well loved Fitbit One (my constant companion for 4(?) years) to my well loved friend. I have "replaced" it with a Samsung Gear S2 (got it free!!). Lots of apps will interface with one another. If you use MapMyFitness it will connect with MyFitnessPal will link up with Fitbit will link up with Samsung Health etc etc. Sorry--I started skimming at some point and this might all be old news. I'm new here, by the way.
The device apps I have used do not have native support for importing step counts. There's an app (Fit and Healthy) you can get (on android at least) that will let you sync your step count from Fitbit to various other device apps and also, if you pay, from other trackers to Fitbit. I haven't paid, yet, so I don't know how good it is. It will not allow you to participate in Fitbit challenges, though. Perhaps those would best be handled on a more general app.
On the confessionals/introductory side of things, I attended my Zumba Fitness instructor class and got certified last month. I'm looking to have that get me exercise and gas money until I graduate (college. third go round. I'm getting too old for this) in 2 or 3 years. I have a ways to go before I realize that goal but I'm totally on it. Burned an estimated 4k calories on top of the basal rate, yesterday, in 4.5 hours of dedicated working out. The stars aligned. I usually don't have so many things to do or places to go.