That depends on a lot of factors and considerations.

• What are the circumstances surrounding your exit? Are they forcing you out because they claim you did something wrong? Are you simply part of a group layoff?
• How big is the company? As a general rule, the bigger the company, the less likely they are to change the terms of a standard severance agreement that has already been through substantial corporate attorney review and approval. Smaller companies may have more flexibility. Those are generalities based on experience ; it varies.
• How big are your requested changes? For instance, are you simply wanting to extend your health benefits for another month, or do you want to demand substantially more money than they are offering?
• What legal claims, if any, do you have against the company? How strong are those claims? If you have an employment attorney, does the company know this? (Those claims are your leverage. If you have an experienced employee rights attorney representing you, it may lead the company to have more concern that you could follow through with a lawsuit.)

There are a lot of other factors that can affect whether you can and/or should try to negotiate any terms of the agreement that the company wants you to sign. This is another “talk to an attorney” situation.