Simple answer... some are and some aren't! It depends whether they have passed a Cross Crew Qualification Course (CCQ). Same as A320 series pilots flying the A330 (as many in the UK do), it is subject to the CCQ course being passed.
A CCQ is required because the aircraft have different type ratings. So where the 757/767 (which shares a type rating) just needs a brief differences course, to go A330 to A340, or the other way, needs a CCQ of just over a week I recall. Still far far less than transitioning from say a 737 to a 747 or a 757/767 to 777 which is a full type rating course (approx. 1 month).
Hope this answered your question!
I used to love the smell of Jet-A in the morning...