Does additional storage have to be USB 3?

I’m considering buying a MyCloud, but I need more than 2 TB.

The information on MyCloud says I can increase storage by adding a compatible USB 3 device to it.

Does it have to be USB 3? Because I’ve got a LOT of USB 2 storage and I don’t care particularly about speed.

Bonus points: can I attach more than one external disk?

Usb2 will work fine, as well as USB hubs.