I think you should split two things:
So create a new button to create a new vrijwilliger. I would use a microflow for that to create the new vrijwilliger object and then show the Vrijwilliger_NewEdit form.
Now how to add that vrijwilliger to the commission. If you place the create new button when you are already in the dataview of a commissie it could be solved by setting the reference directly in the microflow for part 1. Otherwise you need the reference set selector. With this selector you can add vrijwilligers to the current set.
Edit your question if you get stuck somewhere and we will help you further.
Hi atze de groot, you can solve this by right clicking on the input reference set selector
and select generate select page, where a page get generated for selecting the volunteers
for mapping to a particular commission. If there are no volunteers showing in that page then you may add a
create button for creating volunteers in that newly generated page and then select that volunteer .