i have this character that i started modeling using NURBS, after which i convert to sub div and finally to polygons so i can stich up my joints by merging vertices

the 2 methods i know of to create facial expressions are :

1) in NURBs , pull control vertices and create different expressions and after that blend shape but this will all be erased once i convert my NURBS to polygon.

2) in polygon , create face clusters then use edit cluster weight tool to create different expressions when u move face cluster.

my question is : i want to create expressions like opening of mouth and the shock expression when the mouth becomes a O.

this can be done by pulling vertices in nurbs but not polygons.

when i do rigging and animation must my character be in polygon ? can it be in NURBS?

pls advise ( sorry for the lengthy post )

thks