Is there a way of "converting" the MiniBrute to a polyphonic device, using hardware/software or both?
the simplest solution (like dog01 said) is to sample whatever sound patch you did on MiniBrute and then play it through the sampler, polyphonically. That's why I keep the headphones output of the MiniBrute connected to my TE OP-1 to quickly sample some sounds that would sound great as poly (usually pads)
This being the easiest to understand, setup and probably cheapest (there are free sampler VSTs) it's really not the most flexible and respectful of the sound patch because there's a lot of real-time playability that you loose by sampling a "static" sound.
The other solution is to buy as many MiniBrutes as voices you need, and find a MIDI "distributor" that sends each note of your MIDI controller to the respective MiniBrute acting as 1 voice of the multi-voice/synth setup.
One example is to connect them all to a computer running Propellerhead Reason 6.5 or 7.1.1 and have your MIDI controller "play" into a rack extension called Distributor (there's also Charlotte or A-Series 2) then connect each of their virtual CV Gate/Note cables into an EMI (External MIDI Instrument) rack device that would then send them out as MIDI notes to each respective MiniBute's MIDI port.
Quite an apparently convoluted solution but that would allow you to build your own multi voice MiniBrute setup, just like a multi-voice SEM system.
I'm sure there must be an equivalent CV or MIDI "distributor" solution in the hardware modular world, but I'm not aware of it
Edit: oh, dog01 already reference an hardware solution with Mondovox, eh