You can build your own remote volume and seek buttons for the 2005-2007 cars with Nav radio... but it will take significant effort on your part and you would have to find a way to thread the wires, or attach them to the outside of the current steering wheel.
The Nav radio supports this if you have the Nav radio schematic and know which pins to connect to.
I would guess the 2008's do also, but I do not have the schematic.
I looked into it for my 2005 but it looked to me like it was more trouble than it was worth.

I would be happy to build a few button kits for people.
But I do not think most people would want wires on the outside of the steering column.
Plus, does the steering wheel turn more than 360 degrees? I guess I really never measured that.

If there is demand, it will cost more, but it should be possible to make RF buttons that just stick to the wheel with velcro, and you'd need to change batteries every few months. The button case would likely be larger also, and a "reciever" module would have to be built and hidden in the dash.

Anything is possible, but it sounds like a lot of work to me.
What are interested people willing to pay for steering wheel radio buttons?
$250? $150?

If the answer is less than $100 I think you are out of luck, and will have to live without.
I was not keen on paying more than $100, so I'm living without.