If I were you, I would contact them right away via a web form, asking for an extension. Attach a copy of the email maybe in PDF, and a proof that you have requested the Dutch Police certificate, and explain in your message clearly why you need the extension. You don't know how long they will take to process your request so better to ask early on. Also, if they approve your request (which I think they will), you can still submit before the date that they will give you. If they deny (which I think they won't), at least you will be aware of it and be able to try to find a solution like asking for urgent processing of your certificate, if possible.