You cannot do that, as Moodle would not know which activities the user data related to - for example the 3rd activity in the course might be a forum with id 5 on one site, but with id 238 on the other site. In theory, Moodle could try and guess based on activity order, activity type or activity name (all of which could change). In practice that would be very unreliable and Moodle has no support for doing so.
If you really wanted to do this, then you'd need to write your own restore system from scratch (there might be a few bits you could reuse, but all the activities assume that they will either restore just structure, or structure + user data).