Add new instrument

I know how to add a new instrument to a score but how do you add a new instrument to only PART of the score? For instance, a choral piece for 4 voices (say, 160 bars long) may also have a solo voice but only at the beginning of the piece (say, 12 bars). I don't want the empty solo line to continue to the end of the piece. How is the solo line deleted when no longer required?