We should make a tutorial for that... but until we do here's some instructions:
At the top of the gear optimizer above the gearing strategy picker at the middle/top of the page is a link that says "view/edit strategy". Click that, and on the page that shows up is a section for stat weights. Type in the weights you want to use, and the caps you want to use. Then give a weight for the stat after your cap has been reached.
Enter the caps as an amount of rating on gear, e.g. 12000 if you want your gear to have 12000 total haste rating.
Press "Save and Use" button when you are done, and give it a friendly name. Then make sure it is chosen at the top of the gear optimizer page when you optimize your gear.
It may take you some trial and error to get weights that optimize how you want. Hand-entered weights can be tricky to get working well.