I would like it to do +10 instead of +1 per button press (the same for minus)?
To answer the question as asked,
My guess is it would be impossible to hack the car computer in that manner, I'm afraid.
Unless some incredibly dedicated hobbyists (why?!?) made a new ECU chipset or such, it's very unlikely it would be possible to change what the command actually does inside the ECU.
It's easy enough to "muck about" with car ECU networks.
For example, you could make it that for a laugh some other button affects the cruise control.
But my guess is it would be impossible to change what the command actually does when you're actually inside the ECU.
If you're already familiar with low-level programming, here's the awesome article to get you started!
Excellent article on the basics of hacking car networks:
Suggestion, drop an email to the author of that outstanding article. It's very likely they would have a quick insight in to the very specific point you're asking about.
According to the user manual, the + switch has two modes of operation. If pressed, it will increment the speed by 1, if held, the vehicle will begin to increase speed and the new target speed will by the speed at which the button was released. On this basis, I'm not sure that hacking the switch to make a single press +10 would achieve what you want as you'd probably lose the second mode of operation.
It's certainly possible to "hack" it via some advanced low-level programming means, but I imagine that it would be very advanced.
The easiest, safest, thing I can imagine would be using some Arduino or RaspberryPi to intercept and interpret both the button press and brake switch, and use either relays or the boards ports to control the "switch" (meaning closing the circuit). Programming that wouldn't be too difficult, but it would be a bit of wiring work, and you may have to dig into the steering wheel quite a lot. The key to safety would be constantly checking for a brake signal to interrupt the throttle increase action.
The major downside to that would be that if the microprocessor stopped working (for any reason) or broke into an incorrect programming loop or crash, then you would have no cruise control.