Hey Melmac, I have the same situation as you - Kobo and Bookworld accounts with the same sign in details and when I go to link them in Kobo I get the "invalid" message.
I'm not much bothered as I only have a few Bookworld books and they're on my Aura and in my Adobe Digital Editions account - but it would be nice to have them all together in Kobo which I foolishly assumed would happen automatically... oh well...