Do you want

1. a single-staff part throughout
2. a single-staff only at some points during your composition
3. to collapse a two-stave part you already have entered onto a separate single staff?

I'd like to collapse a two-stave part that I already have entered onto a separate single staff.  

Have you tried choosing the grand staff and then using the Implode routine (in the Utilities Menu) to implode it into a separate (single) staff?

That worked.  Thanks so much!