While there are FA's who have done this, it is a violation of AA's health & safety rules (and this a FAA violation) for a FA to comingle food. Once your food goes into the warming oven, nothing else can go there until the oven has been cleaned to spec (which does not happen on every flight).
Even if it were possible, aircraft ovens are not standard ovens and food is generally packaged in specific containers to which you do not likely have access and catering recipes contain more liquid than those served on the ground due to evaporation.
To be frank, I would be just as happy if AA focused on improved cold meals and ditched hot meals on domestic flights altogether.