Thanks so much for the post and for the suggestion. This is definitely something that’s come up before. There’s some related discussion here with links to a couple of (third-party) resources: Importing website layout.
You may also want to check out https://www.wirify.com/. It’s a browser-based ‘bookmarklet’ which aims to convert sites to wireframes. The tool supports export to Balsamiq in the (paid) Pro version.
I’ll add your vote in support of a Balsamiq tool to do this. Full disclosure: right now much of our focus is on getting the new, native desktop apps out of the door so new features like this are taking a bit of a back seat for now.
Thanks again for the feedback.