Hi @Dorian 
Not sure I understand your use case.
You have a WeWeb app and would like to embed someone else’s website in your WeWeb project? That should be doable with an iframe yes.
We have a number of users doing this the other way around with WeWeb apps that are embedded in a Webflow page.
Not sure about this part though: “We would also like to be able to systematically fill out certain fields on those pages.”
I guess that will depend on how they built those forms / fields.