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.

See picture

A lot of the P4 clothing has these extra chunks of mesh, and they have material-names such as skin, set to be invisible. I've settled on Wings 3D as a modelling and mesh-editing program. It's free, there's a good forum here, and it works well with Poser meshes.

Well, blow me down, never noticed that before! You learn something new about Poser every day!

On gap from :

actor head:13
{
name GetStringRes(1024,2)
off
bend 1
dynamicsLock 0
hidden 0
addToMenu 1
castsShadow 1
includeInDepthCue 1
parent neck:13
channels
{
taperY taper

................................................................. 

to :

endPoint 0 0.709 -0.0132322
origin 0 0.62065 -0.0132322
orientation 0 0 0
displayOrigin 0
displayMode USEPARENT
customMaterial 0
locked 0
}