As far as I know, you can't. When you convert from sub-d's to polys you lose your creases, but you can work in poly proxy mode with sub-d's, just right click on the sub-d and select "Polygon" (I think it's called polygon). It might slow down your computer a bit on heavy meshes though.
A little tip: if you don't want the sub-d's wire to show when you're working in poly proxy (I know I don't), just selext the Split Poly Tool, and the only thing left is the poly wire...
Liter is French for 'Gimme some ****ing cola before I break vous ****ing lips!"