Niek, you should be able to add a profile for Adriaan just like you add any profile.
Add him as a child to Johanna's profile (or to Johannes' - just make sure to connect Adriaan's profile to both parents' profiles), entering his name (be sure to use his correct LNAB!), birth and death info, and your source citation.
The system should accept the new profile without any problems. However, if you do have any problems, let us know by adding a new comment to your post.