You can! Since Write.as supports iframes, all you need to do is copy & paste the embed code in that Glitch provides on the project page. To learn how to do that I’d recommend looking over their how-to guide. It’s pretty straightforward.
If you want to see examples of it in action, I’d recommend @triptych’s post here. As a matter of fact, I think he was the first one to discover that you could embed Glitch apps on Write.as.
Hope that helps! I am curious - what kind of app did you think about embedding on there?