The important question here is how the data was imported. To be non-techie for a moment, WordPress works with plugins by what are known as hooks. Think of these as a messaging system. When a new user registers on the website, WordPress broadcasts a ‘message’ and any plugin that needs to know about this event is listening our for it so that it can act accordingly.
Simple:Press is one such plugin. When we get the message about a new user, we create special forum members data and this includes placing the new user in their appropriate forum user group.
So – as I say, it depends how the data was ‘imported’. If it bypassed the actual WordPress code, then the message was not sent and the actions were not taken. So – how was the data imported?