Here is a link to the
StackBack FAQ with more info about their program,
StackBack FAQ
The question and answer that I found most significant was this one:
Can I get rakeback on a poker room where I already have an account?
Most poker rooms don’t accept this; however there are a few exceptions. Contact us if you wonder if the poker room you are using supports this.
Since you most likely can NOT sign up for a rakeback with StackBack for a site where you already have an account, I have not tried to sign up with this program.
Here's another rb forum thread where a rep from StackBack has contacted a few rb members,
????.