You can't. See my response in your other thread.

Another question. A song starts with 4 staves (Sop, Alt, Tenor and Baryton). But the second parts of that song should have 5 staves (Solist, Sop, Alt, Ten and Baryton). How to add only the fifth Staff in the same document?

And the same answer: you don’t, in SongWriter. In the full Finale, you start with all of the staves (or, you can add one in later, but it still goes from start to finish) and then you hide it where you don’t need it.

SW will let you add the staff, but not hide it.

Oh Dear! Thanks.