Temporary extra stave

I am writing out what is mainly an SATB choral piece. However, for about 6 bars in the middle of the piece, the composer asks for divisi, with different lyrics for one of the divisi voices ("Ah..."). I can add the "Ah" above the stave with the main lyrics below, but it would be much clearer if I could add an extra stave for these 6 bars and put the "Ah" part on the new stave, reverting to the basic 4 part score at the end of the section.

So how, if it's possible, can I add an extra stave just for a few bars in the middle of the piece?

Thanks in advance!