Display certain voices and add more later in the song

I'm trying to score a piece for SATB w/ keyboard. The beginning of the song will start with just keyboard and a solo voice(3 stalves). How can I display three stalves at the beginning and then add three additional stalves within the same song? Do I need to staff the whole piece as SATB w/ keyboard and use a feature that deletes measures of rest?

Sidenote: I know I could add voicings on the same staff but, it's easier for choirs to read a staff with their own voice part.