Help: Entities with custom information

Hello,
   I would be grateful if sbdy help me with that:
   I'm trying to put custom information for my application inside some
drawing entities. For example: in an "ARC" entity, I'd like to put a new
custom code: (666 . "CounterClockwise") or (666 . "Clockwise"). I mean that,
whether the 666 code already exists or not, my question is: "Can I add
custom attributes? and how?, and how to access them later?"

Thanks in advance,
   .- Toni

PD: I'm writing it in LISP, in case it is relevant.