I did a little test and it looks like I was able to do it...
The list had a subset of these and I was able to add to it but had to include the items that were already there.
It basically over-writes what is in the option field with your information.
tableextension 50011 "Item_Ext" extends "Item"
{
fields
{
modify("PVS Item Type")
{
OptionCaption = ', Paper, Plates, Film, Ink, Die, Cartons, External Finishing, Finished Good, Cores, Label, Envelope, Board';
}
}
}