Of course you can. That's what MIDI was made for (literally -- the original purpose of MIDI was to allow you to control one synth from the keyboard of another). Just plug the MIDI out of the external keyboard into the MIDI in of the Microbrute. I do this all the time when I gig with my band. There's nothing like playing the Microbrute from a 73 note semi-weighted keyboard :-)
If the Arturia Laboratory keyboard only has a USB MIDI port (as opposed to a standard DIN MIDI out) you'll need to first connect the keyboard to a computer and route its MIDI output through a DAW or similar software to the MIDI in on the Microbrute. Also, you'll need to set the external keyboard to send on the same MIDI channel as the Microbrute is set to receive on.
