You probably are.
But here's the thing: If you getting a severance package, your employer is probably trying to make sure that that's all you get, which isn't unreasonable. Signing is probably a condition of getting your severance pay. If that's more than employment, it's probably worth doing.
If you aren't getting any package... why on earth would you sign anything which would help out the guy that's firing you? Screw those guys.
I'd recommend getting in touch with a local labor and employment attorney. You might actually start with legal aid/legal services, as they frequently help people with unemployment benefits.
This answer does not constitute legal advice and does not establish an attorney-client relationship.