My (2-armed) character normally wears a mithral chain shirt + 1. I'm planning on giving him the vestigial arm alchemist discovery. Is my only choice to sell the armor and buy a new one with three arm holes? Can I simply cut a new hole in the existing armor? Any other ideas?
Vestigial Arm (Ex)
Benefit: The alchemist gains a new arm (left or right) on his torso. The arm is fully under his control and cannot be concealed except with magic or bulky clothing. The arm does not give the alchemist any extra attacks or actions per round, though the arm can wield a weapon and make attacks as part of the alchemist’s attack routine (using two-weapon fighting). The arm can manipulate or hold items as well as the alchemist’s original arms (for example, allowing the alchemist to use one hand to wield a weapon, another hand to hold a potion, and the third hand to throw a bomb). The arm has its own “hand” and “ring” magic item slots (though the alchemist can still only wear two rings and two hand magic items at a time).
I see no reason why it't couldn't be refitted using something similair to the existing rules for Fullplate.
Just grow it out of your hip or head or something that isn't covered by the shirt.
It's magic armor, it should accommodate. Unless your DM is a stickler for random things, don't worry about it.
This is correct. That same armor can be worn by the 5' 4" lithe little thief and the 6' 6" hulking brute barbarian. It's not going to let something as simple as a scrawny extra arm get in the way of protecting your hide. It's magic, that's how it rolls...