Welcome to the Bunnings Workshop community. It's wonderful to have you join us, and thank you for sharing your question about changing a pop-top roof to a hardtop roof on a van.
Before I make any recommendations, would it be possible for you to post a sample picture of the van you're referring to? This will let our members assess the condition of the van. We can then make recommendations on how you can go about converting it. Can you give us more detail as to why you're after a van with a hard top? Are you referring to a converted van that's been turned into a camper?
If you are after such a van is there a make and model you are specifically after? This should narrow down the field and give us a better idea of what kind of van you want to use.
If you need a hand posting the pictures, please let me know.