Facebook was explicit in their reply to Google that they feel they can block people exporting their friend's emails. Friend emails are exported by the "export tool" - in fact, you get but a list of names from that.
I would be very interested in looking at the code and noting people's experience with this. One writer was kicked off Facebook for exporting his address two years ago.
I had a friend create a pipe where he synced his fB to Yahoo, and then Yahoo to Gmail to put his contacts in Gmail... I was contemplating writing a Firefox (or Chrome) extension where you input your Yahoo / Google / Facebook account info. and the extension goes off and does the piping for you. But! this looks awesome, need to go test (by reactivating my fB account...)
Regardless, this sounds like a great extension and you should not have to open a mail account with Yahoo or MS to export your friends. This is an excellent idea!