In some cases - check this well in advance - the contribution for supplementary pension accrual is deductible from your gross income. In that case, you will have to prove that you need the extra insurance because otherwise you will not have a high enough pension (e.g. because you started working at a later age or did not work for a while).
The website of the Tax and Customs Administration features a tool to calculate whether you have a deficit, under ‘annuity premium calculation tool’. You need to have a number of details at hand in order to fill it in properly, including your income and the value of the pension accrued with your employer. You can find this information on the uniform benefit statement (UPO) that you receive each year from your pension insurer. If you do not have this statement, you can ask for a copy. Your pension provider is obliged to send you a copy so as to ensure you can always check how much pension you have accrued.
Please note: The Tax and Customs Administration has set a maximum for the supplementary contribution that you are allowed to pay in. Anything over and above that cannot be deducted.