I don't have PPRo in front of me while at work so I'm assuming it's not obvious.
havign said that, why do you want to do this? What are you doing that needs it.
I'm asking this because unless you're goign to write a lot of text, having many words on a line the effect of justifying both left and right will just look 'wrong'. Even the amoutn of text used for DVD subtitles is not really enough, in my opinion' to warrant this. Any fount size smaller is going to be unreadable as well.
I suggest you check (in Word if necessary) what thing swill look like and confirm you really need it.
in the meantime, I'm off home now so will hopefully have time tonight to have a play and see if it can't be done anyway - if for no other reason than "I just want to ok?"