is there a script for boarding vehicles?
or is it too complicated for CE's engine?

You don't just need a script, there's a lot more involved in getting it to work. CAD made it work, but it wasn't like Halo 2. When you tried to board the vehicle, it kicked the person out, and you get in once it hits the ground automatically. In H2 you just board it. And it only worked on Single Player.

You need to modify game engine using the source code

No, you don't have to for Single Player. If you want it to work like Halo 2, then yes. But if you want it like CAD had it, you don't need to. It just requires a lot of scripting, tagging, and animating.

Cool! Did he upload any video of it?

Killa is a good scripter, so I'm going to say he made this one by himself and CAD isn't the only one. Anyhow, it's still a video that shows how it works: http://www.youtube.com/watch?v=_nI8cWoVKyI&feature=channel

There is a vehicle boarding video on youtube somewhere...

Damn it it's not avalible in my country. >:(

this is how you do it. you out a seat on the front of the ghost for example. not valid without a driver, so you dont board unmanned vehicles, then "if player0 is in this seat, start animation "boot the ass thats driving out of the seat" sleep for however long it takes to get to the point where the foot makes contact with said ass in the driver seat, then vehicle unload magic or something on the driving encounter, vehicle load magic player0"
the only thing that has to happen for this to work is that there is only one character in the encounter who is driving, otherwise the whole encounter gets booted out of their vehicles. simple enough