That is just a matter of finding the right sample or soundfont you want. You can search through the various free soundfont sights like www.hammersound.net and www.bananaquarts.com and see if any of those strike your fancy. I'm sure there are some decent free choir samples out there. There is a crappy one in one of the FL sample folders. If you don't have any luck there, you might try some place like www.bigfishaudio.com and lay down a little cash (or a lot) for some high quality samples. Some of the online sample dealers will even sell individual sample downloads from 2 to 10 bucks each.
Hope that gives you a start.