Yes you can. I can't remember the exact details, but I created a new project with CSS adapters and saw what was added and then added those things to my existing projects. It's a matter of adding the adapter files to the App_Browsers folder and App_Code\Adapters, which you'll have to create those folders. And there may be some lines to add to your web.config. So, it can be done. I actually didn't add all of the adapters, I managed to just add the adapters I needed. The adapters work great at allowing designers to apply styles and it also reduces page size because it builds your components with <div> instead of the more verbose <table> tags.
Jarell Lloyd
Senior Developer
Xede Consulting Group