I was wondering if there would be any answers about this. Without that feature, it's a one way tool: all changes must be made here, the compiled product can't be usefully edited.
I imagine it would be pretty challenging to parse an existing website into something usable, but it would definitely make the tool a lot more useful (not to say it won't still be useful to lots of people).
IIRC from the video, at Macaw's core is an embedded webkit engine so they might not have to do any parsing at all but instead interface said engine. That being said, I'm not quite sure how difficult it would be to go from having it parsed to making something that can work with the parsed code since it might have a structure that would have to be further parsed to extract the meaning of the code.
I imagine it would be pretty challenging to parse an existing website into something usable, but it would definitely make the tool a lot more useful (not to say it won't still be useful to lots of people).