Yes, negotiating your settlement agreement is not only possible, but often wise. Employers usually initially offer a standard proposal, but there is often more to be gained from it. Think of a higher severance payment, relief from work, dropping a non-competition clause or reimbursement of legal fees. With the help of a VSO lawyer, you will be stronger at the negotiating table.
A good example of this is the story of Jan Onderhandel. With legal support, Jan managed to increase his severance pay to twice the statutory transitional compensation and even received three months' extra salary.