Can you create an assassin character in Prophecies or Nightfall?

As a noob to GW I may be missing something obvious.
I can't work out how to use the expansions classes (I have all three games) of Assassin, Dervish, etc in the other expansion packs.
Is this right? Are the new classes only useable in the specific GW game they are launched in?

Thanks for any info

No, you can use all 3 in all three games, although i am also a noob so i cant tell you much more.

You can only create them in their chapters that they were released in, but once they are created, you can take them to all of the games

There not expansion packs.

When it comes to the campaign specific professions, you can only create them in the campaigns they were introduced in (sins and rits may only start in Factions; dervs and paragons may only start in Nightfall). However, you can take characters across campaigns after you reach a port and unlock travel with quests.

The ports are at Lion's Arch (Prophecies), Kaineng Center (Factions) and Kamadan (Nightfall). Take a character to any of these towns to find the quests that will let you cross between continents.

It also fits with the storyline connecting all three. The Assassin and Ritualist professions are sort of closely guarded arts of Cantha, same with Elona's Dervish and Paragon. Plus, in the time you begin Prophecies (2 years prior to the main events - 5 years before Nightfall) Ascalon won't have had much contact - if at all - with Cantha and Elona (The Canthan Embassy in Ascalon was shut down by then, I think.)