I think what you're seeing is part of Posette's neck, not the catsuit neck. I've just looked at the catsuit just now and I can't see any extra stuff above the collar.

If you want to stretch the collar of the catsuit itself, there are plenty of tutorials on making morph targets at Renderosity - search for Dr Geep's "Morph Targets with Wings3D" tutorial.

The basic process is -

1. Export the neck as an .obj file with "as Morph Target" checked

2. Alter the neck in a modelling program

3. Import the morph target by going to the parameter dials and clicking "Figure/Load Morph Target" and selecting the new .obj that you made in the modelling program.