adding voice parts

Please clarify what you mean by that, 'voice' unfortunately has multiple meanings, as does 'part'

Do you want to add another staff representing what a vocalist would sing? See the handbook: Change instrument set-up

Or do you just want to add lyrics to an existing staff? See the handbook: Lyrics

Or do you want to add more notes to an existing measure, "in parallell" to the existing notes, but with different rhythms? See the handbook: Voices