You're developing a production number that's a whole new scene in itself.
No worries - just add it in Scratch.
For example, if you're so inspired to make "Ding Dong The Witch Is Dead" a whole new scene right after "Munchkinland", just tap the blue "Edit" button at the bottom right corner.
You're now in Scratch mode, where you can edit scenes, acts, and the details within.
Tap the red plus (+) button between "Munchkinland" and "The Corn Field", and on "Add Scene".
You've just added a new Scene, labeled it "6A", and titled it "Ding Dong The Witch Is Dead".
Now add the characters you want in that Scene, and hit the blue "Save" button on the bottom right corner.
You now have a new Scene to add that Munchkin choreography video to!