I've just tried this - when I try to upload my new zip file via the Export Manager I get the following error:
Unable to import file. Only cordova projects exported from C2 or C3 can be imported.
Any ideas? Do you think it's because I'm working on a Mac rather than a PC?
Hmm it is possible that a mac makes zip files a bit differently I guess, I'll test it later on my Macbook Pro and post here to let you know how it goes. Gimme a couple of hours and check back :)