Need some help with Modding Items

Basically I want to be able to create more "slots" for items

For instance, I know armour (chest armour for instance) is usually a chest piece, some shoulder pieces and often an undershirt and or a set of upper arm armour.

I know the chest piece has the tags    

<Type>Torso</Type>
<Type>Surcoat</Type>

And that these tags tell it which pieces it cannot be used at the time as.

What I want to do is add in some more types (shoulder, Bicep, Tassets (the skirting on the bottom of a breastplate))

I can change the armour to have the types but I can't seem to get it to appear in the sovereign creator.

Am I approaching this the wrong way? Do I need to define the additional types somewhere or is it not possible?