Everything's negotiable, but the wisdom of trying it depends on the strength of your negotiating position. Many severance agreements simply reiterate obligations to which you've already agreed in employment/non-compete/non-solicitation agreements. If this is the case, you have basically no leverage. If they're asking for substantial concessions (say, you have a 3 month non-compete and the severance agreement extends it to 12), you've got more of a shot.