Depending on your needs it can be easy or a bit more complicated.
Do you want to have a single MIDI track in Reaper sending and receiving MIDI to/from Maschine-VST by using different notes to trigger different Pads for one-shots?
Or
Do you want to 1 MIDI Track in Reaper Per Maschine Pad with the possibility of each of those MIDI tracks playing each Pad Chromatially?