Attaching the ped to an element will effectively keep it in the designated place, the 'updatePosition' parameter will not work. I haven't seen yet a solution to this (peds jacking cars or even peds entering a vehicle using the normal mechanics); I think it can be done, though likely would not look as good as in SP, since cutting some corners would be necessary.
You need to calculate the positions required, make sure the vehicle is stationary (maybe even freeze it for a short while), make the ped walk to the car door and play the animation, you open the car door by script (on a loop if you want to see actual motion and not simply 'snap' it open), warp out the driver, move it in the right position and have it play the animation, finally warping the hijacker inside.