The MST MIDI to CV Expander module is a tiny but extremely powerful expansion module for the MST MIDI to CV Converter. The Expander includes an arpeggiator and allows you to have additional CV, gate, velocity, modulation and aftertouch outputs on a separate MIDI channel. You can also set it to the same MIDI channel as the Converter and use one of the three note priority selections, which will allow you to do duophony/pseudo-polyphony. You can daisy-chain Expanders together to add even more channels.
The Expander really shines when it is used with a DAW. Multiple Expanders can be set to different MIDI channels in any sequencer (like Ableton for example), making live performance with your Eurorack simple.