If you want to take your PCs from Oerth to Athas (the Dark Sun world) then you have two options for the transition - spelljamming or planewalking.
The Dark Sun crystal sphere wasn't documented in the original Spelljammer boxed set, but The Complete Spacefarer's Handbook says that its crystal sphere is "not on the spacelanes where...Greyspace can be found" and says that the current inhabitants of Athas have no knowledge of Spelljamming.
The Complete Spacefarer's Handbook recommends that Athas is not used with Spelljammer, but I don't see why you should let that stop you if you are careful.
Beyond the Moons (the official Spelljammer website) has a Dark Sun/Spelljammer crossover setting called The Crimson Sphere. Don't forget that knowledge of Athas should be almost impossible for your players to obtain. Oerth is supposed to be a groundling world, so the people living there should know little if anything about wildspace and spacefarers.
You will need to send your players up into Greyspace, but even there they should find it almost impossible to find information. A journey to Athas should be a long quest that is interupted by trips to several other crystal spheres where the players seek out people who might be able to help them.
If you want your players to planewalk then you will need the Planescape setting or the Manual of the Planes.
You will need some sort of clue that points the players towards their destination. A powerful magic item (especially one that binds itself to one of the players in a manner similar to the apparantly cursed cloak in the Spelljammer books) could be something that makes the players want to know more.
I suggest you go through your Dark Sun products and find a suitable item. Maybe you could give one of the players a defiling item that destroys plants when it uses its powers. If you stuck a druid with an item that used defiling magic they would probably want to be rid of it.
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