Set a reference with Modaliteit so you can use the reference to retrieve the NewPrognoseJaren in the form.
Relate the new object to the object you are sending to the page, then on the page retrieve the related object
Why would you pass 2 parameters to a page if they are not related? Usually there is some relation, or you might think about redesign the functionality in two pages...
What you can do if you don't want the two have a relation is create an extra non-persistable entity used for this page that has an outgoing relation to both entities. In that way you don't have to store a relation in the database.
I assume your prognosejaren entity is non-persistant and you cannot refer from (persistant) Modaliteit to prognosejaren because of that? An extra non-persistant entity with two outging relations will help you in that case.