AI, which is not guaranteed 100% accurate, suggests...
"In Crescendo you can add a custom MIDI instrument by configuring a VSTi or a MIDI instrument mapping for the Parts Setup, then selecting that instrument for the desired part.
What to try
Open the Score toolbar and choose Parts Setup (Parts & Staves).
For the part you want to use the custom instrument, choose the instrument type as either VSTi (if Crescendo supports that in your edition) or MIDI input device.
If you’re using a VSTi: pick the VSTi option, then choose your custom instrument from the available VSTi/instrument list. You may need to load or register the VSTi in Crescendo first, per the VST plugin instructions in Crescendo’s help.
If you’re mapping a custom MIDI instrument: ensure your MIDI device is selected, then assign the correct MIDI channel/note palette to the desired staff position. Crescendo lets you map each note position to an instrument in the Drum Kit Map dialog or instrument map for percussion, but you can also map to standard MIDI instruments if the software supports it in your version.
Save the mapping as a preset if Crescendo allows, so you can reuse it for future scores.
Tips
If you don’t see an option to add VSTi or map a MIDI instrument, check the edition you’re using (some features are edition-limited) and verify in the Help/Manual under Parts Setup or Drum Kit Map Dialog how to link a custom instrument to a staff."