Hello – is it possible to have more than one Dante card in a single H9000? I already have one Dante card and a MADI card, but the 3rd slot is empty.

This should work fine.  The cards will appear as two separate 32 channel Dante devices on your network.  As you probably know, the Dante ports in the back are in pairs and each pair is connected to an internal network switch, so you can daisy chain the two cards (you don't need to run two ethernet cables back to your main switch).  How is your studio's clocking set up?  The most common configuration in a pure Dante setup would be to let Dante take care of the clocking and set the H9000 clock source to follow one of the two cards.