For my first message here, sadly I'm a bit too late !
I'm French and a member of the BMW airhead forum "Le Flatistan" where you made an appearance few months ago.
I'm sorry to tell you that you have been misinformed ! It should have been possible to register your bike, with a "certificat de conformite" from BMW France (100€), an original rear wheel to pass the "contrôle technique" (our MOT, but quite easy to trick even with a modified bike), your original paperwork for the bike and an import tax receipt (and the relevant taxes paid).
I'm so sorry for you, and invite you to ask, even in English, on Le Flatistan for this kind of problematics.