Assuming you mean an additional instrument, you'd add it via Edit → Instruments.
Then you'll likely want it hidden when empty, which you can do via the Staff/Part properties of it; or if it should be applied to all staves, by using Format → Style → Score
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Assuming you mean an additional instrument, you'd add it via Edit → Instruments.
Then you'll likely want it hidden when empty, which you can do via the Staff/Part properties of it; or if it should be applied to all staves, by using Format → Style → Score