If your hub has only thinnet 10Mbit/s specially marked backbone port, you may use any of your free Fast Ethernet ports for backbone connection. Just do not forget to use crossover cord to connect it to another hub.
It's not wise to have backbone slower than stations connection, you're right. So if you don't have free Fast Ethernet ports, you'd better unplug one of the stations, just because backbone is more important.
With best regards,
Dmitri Abaimov, RCDD