Figured it out! 
Yes, you can embed a Quizziz, though it needs to be assigned as homework and get the join code for your students. You will not get the analytics for the quiz on Formative, but rather on Quizziz.
Of course, your embedded Quizziz may be part of a complete assignment.
I created the one below using a trivia quiz and in it a give a brief explanation of how itās done. Feel free to play:
https://goformative.com/clone/QWCUSN
@smclaughlin I too use several platforms. The benefit of embedding, in my opinion, is that you can have everything in one place, including your Quizziz and other resources. For my middle schoolers this means minimizing distractions from āother searchesā.