I need to change the marked section to Percussion 1 line. How can I do that?
1 line.dorico (1.6 MB)
I think you will have to give your piano player a second instrument (the percussion). You know how to do this, it’s in Setup Mode. In your score, instead of using the Ossia, put that music into the percussion player. I am sure you can figure it out 
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You won’t get a one-line staff as additional or ossia staff of a piano.
instead, add an instrument with a custom staff size:
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Do I need to add one more instrument?
Yes, I did it here with a snare drum — see the screenshot above, or the file here:
1 line.dorico (1,6 Mo)
Thanks.