Hong Kong Agriculture Fisheries and Conservation Department web site will have details on required permits, any potential quarentines, health requirements, etc. - http://www.afcd.gov.hk/english/quara...a_ie_ipab.html
Pet relocation agent (can help with import permit, pickup from airport and getting cats through customs, booking flights if needed, etc. -
http://www.export-a-pet.com
A quick chat with your vet will provide good details on the safest and most comfortable way to actually transport the cats on the plane. I think they will have to go in the cargo area, but check with individual airlines for their policies on international pet travel (there is a special area for animals down there). Generally, animals should be shipped in a travel crate large enough for them to stand up and turn around in, but not much bigger (don't want them to move and jump about or get tossed around by the plane, which can cause injury). Each cat should go in a separate crate. Line the bottom of the crate with an absorbant material (like shredded newspaper or a couple blankets/towels), put in a water bottle (tested beforehand to make sure its not leaky), and don't feed the cats on the flight or 12-24 hours before (check with your vet on the feeding requirements though). Generally I don't think vets recommend sedation during the flight, but again, check with your regular vet.
Good luck.