Yeah, it's really easy to add them. You can get a kit from Turner Motorsports for like $75 and it includes the plastic shifters and all. Then you just screw them into the spaces alongside the steering wheel, there should be some holes about 1/2 inch on each side.
Then you need to go into the idrive system and turn on paddle shifting, which is somewhere in the menu but I forget where...
Nothing about BMW's allow you to "just buy them separately and add them just like that." Just to add an ipod adapter is like $700 installed from the stealership, and that's just running a few wires from a pre-made kit. So, no, you cannot just add paddle shifters.