liturgical unmetrical

To add staves, you would normally do so when first creating the score, by choosing the appropriate instruments in the Create New Score wizard. If you have an exisitng score you wish to add staves to, use Edit / Instrument to redisplay the same dialog.

It's possible you don't really want more staves - to be clear, four part chant would normally be done in two or four staves total - but instead, more *systems*, which is to say, more measures. In which case, simply add the measures via Add / Measures. To create the effect of unmetered music, combine measures via Edit / Measure / Join Measures (also Split Measure) as needed.