How does a pilot for a 135 opps airline get CAS certified to be able to fly in the cockpit of a 121 opps ?

My airline is a non-cas airline, I want to know if I am able to be cas certified by the faa without going through my airline so I can ride the jump seat on far 121 carriers.

I think this explains it..

http://web.nbaa.org/member/ops/part135/wetlease/A00820060801.pdf

I don't see any reason why you couldn't be CAS/SEP certified in a 121 carrier as long as they are willing to let you ride to obtain your cert.