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 where your existing bot lives.
Send a Re-Engage -previously known as Broadcast- from the existing bot and ask users to interact back. All they need to do is push a button, type something in, or similar.
Those users who interact with the page will be captured by the empty bot, which will enable you to Re-Engage with them from there going forward.
Deactivate the old bot and use the new one.
Note: You will lose those users who do not respond to the initial Re-Engage message, so make sure it encourages the users to reply or tap a menu button.