Hey @StefanMusch
If your plan is to delegate DNS to Netlify for bouwbureaubrok.nl you will need to move all existing records to Netlify first. Read [Support Guide] How do I migrate a domain to Netlify DNS with zero downtime? for more information on this.
If you wish to keep DNS for bouwbureaubrok.nl external follow the Configure external DNS for a custom domain documentation—you don’t change NS records in this instance.
If you have any further questions or issues, please don’t hesitate to reply.