As it is not possible to export users from most existing platforms at this time, here is a workaround on how to import active users in Chatfuel.
Connect an empty Chatfuel bot to the same page with your existing bot.
Send a Reengage message from the existing bot and ask users to interact back. All they need to do is push a button, type something in, or anything similar.
Those users who interact with the page will be captured by the empty bot, which will enable you to Reengage with them from there going forward.
Deactivate the old bot and use the new one.
📌 You will lose those users who do not respond to the initial reengage message, so make sure a message encourages the users to reply tap a button.