I do not know of any treadmill that integrates directly with Fitbit. iFit prides itself on being a one-stop-shop for NordicTrack/ProFit users, but unlike other companies that provide both the equipment and the online presence, like Fitbit, Polar and UnderArmour, iFit doesn't have any clue that people use all these devices and so the apps have to talk to each other to get all the data in one place.
The only thing I can think of, without iFit's actual cooperation on this matter, would require you to be fairly computer-savvy; you'd essentially develop a small desktop application whose purpose in life is to regularly log into iFit's API (for which documentation is sparse to nonexistent) and pull recent workout details synced to iFit from your equipment, then upload those workouts into Fitbit using their public API (for which documentation is much better). The app itself would be fairly straightforward for a career Java or C# coder to set up; the big hurdle is figuring out iFit's workout API, because they don't make it easy.
If you want to look further into it, and as a reference for anyone more coding-savvy who finds this thread:
iFit has an example site at http://ifit.github.io/active-dashboard/, which uses an API endpoint https://api.ifit.com/v1/activity_logs to retrieve iFit's workout data. You can use your browser's developer tools to see the calls it's making and replicate them using the web request tool of your choice.
The site uses OAuth 2.0, which requires a manual login and approval to let the site/app use iFit's data. You can poke around with setting up a browserless "two-legged OAuth" authentication (no guarantees), or hack the UI-based authentication by scraping the HTML of the OAuth page for the OAuth secrets (the big one is the _csrf variable, which is a random hexadecimal-encoded value provided by the OAuth server to identify the request) and returning them with an "allow" decision as a postback to the same OAuth URL. The iFit authentication endpoint is https://www.ifit.com/api/user/login, which you will need to log into with your credentials before attempting to set up the OAuth session at https://www.ifit.com/oauth/authorize. The "Location" header of the response will include the token and its expiration timeout (I believe in seconds from the approval time, as that works out to a one-week valid span assuming you don't manually log out of iFit), and you simply use the token with an Authorization: Bearer header on calls to the Activity Logs service.
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