Yes, cassettes can be referred to as indoor units (they're a subset type of indoor unit). I don't believe that you can currently add more than one terminal (indoor) unit per zone, so you'd either have to divide up your thermal zones, or create a fictitious aggregate cassette that mimics the performance of the smaller units.
The recently added OpenStudio VRF drag-and-drop capability (see here) allows you to pull in actual VRF components from TPEx via BCL. LG is currently the only manufacturer to provide data sufficient for modeling, but they have contributed their entire VRF Multi V line, which includes 52 outdoor units and 79 indoor units (of which 21 are ceiling cassettes).