Samsung and Verizon seem to think they know best, so you can't change anything in the Desk App that comes with the phone. Since I use my desk dock as a bedside alarm clock, I downloaded Alarm Clock Plus which is a free app. It allows you to reprogram it's app buttons. The only drawback is that the settings to dim the nighttime screen saver clock are not very intuitive. I was finally able to figure out how to get it to an acceptable dim setting, but it was a PITA getting there. Otherwise, its a good free app.
If you use your dock for more daytime uses, you might have to try some other market apps. BTW, I encountered the same lack of options with the stock car dock app. I found a fully customizable App for it too: Car Widget Pro. You have to dedicate one of your home screens for the full widget, but it works great.
Rich