You need to set the pads to the same channel as your Impact. As for the notes to set your BSP custom drum map to, the Impact follows the regular standard of starting a percussion map at MIDI note number 36 for the first kick, then continues up the instrument list from there (have a good read of the manual for finer details and how to change the Impact note map). Set the MIDI note numbers of your custom drum map to 36 and above and assign them the same MIDI channel as your Impact, and that should have you running playing the instruments on the Impact with your BSP no worries!