The "CJ multi-crew ops" question of collecting an ATPL has come up so often that it is actually one of the small number of FAQ's directly addressed on the UK CAA web site, and is also covered in the LASORS publication. It's also come up several times before here on PPRuNe. Nevertheless, repetition is the very soul of the Internet, so, once more with feeling:
The correct answer is that to get a JAR ATPL you need, amongst other things, "500 hours Multi-Pilot operations on aeroplanes type certificated in accordance with the JAR/FAR-25 Transport Category or the JAR/FAR-23 Commuter Category or equivalent code, or single-pilot aeroplanes operated by 2-pilots according to operational requirements.", and you need to pass a check ride in which "The applicant shall demonstrate the ability to perform as Pilot-in-Command of an aeroplane type certificated for a minimum of two pilots under IFR, the procedures and manoeuvres required with a degree of competency appropriate to the privileges granted to the holder of an ATPL(A)".
Both quotes directly from the UK CAA.
So, if you are flying on a CJ family aircraft (or another single-pilot aircraft, like a Premier 1), but are operating it for public transport under JAR-OPS1 which REQUIRES you to operate multi-crew, then the time you were REQUIRED to operate multi-crew counts towards the 500 hours, and satisfies the FIRST requirement listed above. Note that the official interpretation is that "elective" two crew operations don't count, so if you are a pilot's assistant on a private single-crew operation, that doesn't count.
To tick off the SECOND requirement listed above, you need to pass a check-ride in a MULTIPILOT aircraft, and the CJ family does NOT count (including the CJ3 - it is nothing to do with weight). You therefore would need a second type rating in an aircraft that can only be flown multi-crew.
But, since you can count your CJ multi-crew time, it would be considered quite normal for a CJ rated pilot with more than 500 hours multi-crew on the CJ and a frozen ATPL to automatically collect their JAR ATPL as soon as they add a type rating for a multi-pilot aircraft.