You can add patches you've created or purchased from other sound designers as well as third party plugins like Keyscape or Omnisphere to Sunday Keys and MainStage.
You can map these into the "Extra" section of Sunday Keys quite quickly:
- Select the on-screen knob in the "Extra" section.
- Press the "Map" button from the Channel Strip Inspector window.
- Click on the appropriate channel strip volume fader.
- Make sure to set the "Max Value" for the knob to an appropriate amount. We recommend no more than 0dB for most channel strips.
Mapping sounds to the Extra section and making more complex mappings to one of the full sections of Sunday Keys are covered in Video 11: Using Any Other MainStage Patch in Sunday Keys on the Sunday Keys Resource Page: