Chapter 4, equipment, in Armour section:
"For comfort, and as part of its protection, all armour is considered lined or padded."
and
"If a character is wearing multiple pieces of armour on a location, only the highest Armour Point score is used"
So basically, every chainmail already includes padding, which probably gives one of its total armour points. And you cannot stack them anymore by standard rules. In theory one could put linen armour on TOP of his chainmail, but I don't think that would be practical.