There is not a way to add a staff in the middle of a score. There is however a way to only have the new staff show when there is notation in it. You can use the "Hide resting staves" function though. This feature is described in detail on page 11.5 of the User Guide. Here is an exerpt from the User Guide:
"If you have instruments that are tacet for sections of a full score, you can
choose to hide them. You have full control over when and where they are
hidden, so you can for example have all the instruments showing on the
first page of a score, and then custom instruments hidden or shown
throughout the rest of the score as you choose.
1. Select the region you wish to hide empty staves
2. Go to View>Hide Resting Staves
3. To show resting staves for specific instruments (maybe to make full
use of the page, or to highlight that the clarinets are entering over the page
for example), make a selection of the score, then go to View>Show Resting
Staves. Then select which instruments you wish to show within the selection."