There's no conversion between JAA and FAA at all. The only thing that transfers is logged flight experience. I'm not sure what the FSDO told you at first. In any case once you have FAA certification you have to add ratings in the "normal" fashion - in your case a type rating ride. At least I've never heard of "upgrading" a(n) SIC type rating on the basis of another type rating (767) on your license - which might be what confused the FSDO guy. It'll be the same for getting your JAA 757/767 type; you'll have to go throught the "regular" JAA process. There's an agreement between FAA and Canada (see above) but that doesn't help you.
Now one option on the JAA side is to find an FAA inspector or DPE who can sit in on your next PC and possibly issue the rating based on that check.
I hope this helps. Really would have liked to have heard your conversations with FAA. Maybe someone more knowledgeable will chime in.
Under FAR 61.75 one can get a private pilot license based on a JAA license, but it doesn't apply to you since you are already FAA certified.