Does anyone know of an aluminum threshold that would fit over an existing wooden one? This is an entry door for a house that is about 45 years old. The old frame is bricked in and would disturb the mortar if an attempt to remove and replace would occur. Im thinking od a threshold that would "cap" the old one with a themal break built into it.

Never seen one like you are hoping for. The aluminium threasholds are structural to carry a persons weight who may step on them, they are not hallow and aren't like a cap. I would use a reciprocating saw to cut vertically down on each side as close to the jam as yopu can. Pry out the old threashold and cut your new one to be a tight fit.