The advanced heavy armour includes a breastplate, a backplate, and some other plates for extra protection. Also it includes a chain mail coif that protects the upper chest area.
Uses only ingame textures, it is meant to be used as an attachment.
Well as far as I know, it is impossible to make a pants attachment for the normal models.
That is because we don't have any "leg,left" or "leg" attachment point. So if you do a model of a pants and lets say attach it to the "chest" when the unit moves the pants won't move with the legs. They will remain still and the legs of the unit will go threw them.
In my opinion the only way to make attachments pants possible is to add 4 more attachment points to a unit and create separate parts of the pants for it.
Sorry, I wish it could be possible.
Edit: I am testing if its possible to do gauntlets. I know you can do boots but gauntlets is anther story.