Importing users SHOULD be relatively easy depending entirely on the forum of the exported data. CB has built in importing/exporting functionality for users directly in CB > User Management (top right of the table). I recommend exporting 1 user so you can see the CSV format. You'd then need to format your user data import to follow that CSV. Next you'd simply import it and it should create your users.

Activity might be a bit trickier. I've no idea how activity is stored on that site, but you'd first need to import your users. Then you'd need the activity on that site mapped to their new Joomla user id. You should then be able to just directly import activity into _comprofiler_plugin_activity. My guess though is how activity is handled will be WILDLY different so this probably won't work so well for some activity.

Additionally if you could provide an example user and activity export (structure and data) from your existing system I can try and suggest a more specific import strategy.