To add new Classpads to your Classwise kit:

Place the Classpads in the Docking station and pair the full set again automatically,

or

pair individual Classpads manually.

What’s important, you can add and mix different sets of Classpads previously paired with different Control hubs.
Keep in mind, however, that the Control hub remembers the last set of devices it was connected to.
If you switch to a different kit or pair the Control hub with other Classpads, you need to pair all of them again.