Actually, I just discovered that you can't. But that's not what Kobo reported on their blog:
Great news for all of you that have the Kobo eReading app on your iPhone or iPad – you can now bring in books from other eBook services to your Kobo library! And, for those of you that have an iPhone or iPad and haven’t downloaded the Kobo app, here is just one more reason why you should.
Like I reported last week, the ebooks you add this way will _not_ be synced to your Kobo Library (on the website). They'll just be on your device. It's not the same thing, IMO.