How can CP9 members and Sanji run in the air? How can CP9 members and Sanji(post-timeskip) run and stand in the air? Do they use something like Haki? If not, what kind of techniques do they use? If it is Haki, can Luffy and Zoro run and stand like they do?
Actually this technique that Sanji uses is called 
Sky Walk   it doe not use any haki, he just learned it by training while running everyday increasing his speed making him run in the air.

Wiki states:  


  This is the aerial version of Blue Walk. Sanji kicks the air to jump higher, giving the appearance of flight. Sanji created this technique during his ordeal in "hell" to escape from the okamas. It was first seen used to escape after being surrounded by the Sea Urchin-Armor Spine Squad, making them hurt themselves. (Vol. 64 Chapter 635 and Episode 555)


As you can see he developed and learned this technique because of his ordeal during the time skip while training in Kamabakka Kingdom. giving him immense speed.