Severance is never a requirement of any employer unless you have a signed employment agreement stating otherwise, or, it is a written policy of the company.
All you can do is ask. They'll probably say no but you're bound by the companies severance policy/program but legally it's not a required benefit. BUT keep in mind the company wants you to sign that agreement at the end of the day so you could get more. You could try and use trigger words during your conversation.
Under what circumstances can a final paycheck be withheld under Illinois law? There are no circumstances under which an employer can totally withhold a final paycheck under Illinois law; employers are typically required to issue a final paycheck containing compensation for all earned, unpaid wages.