It's possible, but it will never seem realistic (the best you can do is to show like MDK2 (models move head/mustache then in the bubble speech they have 1 small head wich shows the mouth animation) or like in HL (the mouths simply open/close but show no reaction, just like a ventrillic doll (you know, those dolls that you sit them on your leg, have your mouth closed and make it look like its the doll speaking insted of you)).
You will need to make each animation, then in FPSC make a script so it uses correctly each animation.
Really hard to do, and way to much time spending, but, 1 thing is for shure, if everything is sincronized correctly, it looks way cool and will give a really big thumbs up to your game rating.
EDIT: The way i say it will never look really realistic is that, for a nice face expression you need to have a "highly" detailed head (more then the normal), and most of our members are having trouble having a nice detailed level, if you spend to much detail on NPCs, you won't be able to use much detail on the levels, either way, the game will be unplayable.
I gave 2 small game exemples, wich you can google in the case you don't own the games, so you see how they made it, it's really a simple speech type and it doesn't require lots of "head details + animating", also It's always easier to see life exemples that passing everything to text, isnt it?
My love is seperated in 2 parts, 1 of them is 3ds Max 7, the other; FPSC.