There are a few possibilities, depending on how specifically you want the tracks to work with respect to both MIDI control and audio output.
If you’re content to have this ensemble come out of a single stereo mixer channel, you’d just need to create additional MIDI tracks, including configuring which specific MIDI channels each MIDI track uses, and making sure the MIDI channel configurations for each instruments in HALion Sonic match the separate MIDI tracks you want to drive them. Then you would use HALion Sonic’s Mixer tab to deal with any balance, panning, etc. differences between individual instruments.
If you also want each instrument to come out of a separate MixConsole channel, then you’d need to configure multiple audio outputs in Cubase and route the individual instrument outputs within HALion Sonic to come out the relevant outputs. There is an excellent video on how to do that with HALion Sonic (and also Groove Agent) by Dom Sigalas at:
One other alternative is to use a separate HALion Sonic instrument track for each of the instruments you want in your ensemble, so basically doing your usual double-clicking on HALion Sonic in Media Bay one time for each instrument you want in your ensemble. (Or add multiple instrument tracks from the menu to get those tracks appearing more quickly.) Then rename the tracks to match the instrument names you want (to avoid confusion later), and go into each instance to configure the Iconica Sketch instrument you want in that instance. (This is the approach I’ve used with the projects I’ve done with Iconica Sketch to date. Though it obviously uses more instances of HALion Sonic, it feels simpler with respect to organization of my project.)