Unfortunately at this time there is no way for accounts without rakeback to get switched to a rakeback affiliate. I am in the same situation as you, where I created my account without rakeback and now want it. They have been claiming for a long time to be working on a solution for providing some sort of compensation to non-rakeback players but there is still nothing significant, just Iron Man which is 2-3% at best.
The best option you have is to email
http://www.rakebackpros.com/ and say that you want to be put on the waiting list for rakeback. That site is run by Full Tilt and is currently the only way possible to get rakeback if you don't already have it. They generally won't switch over any accounts that are linked to an affiliate already.
Personally, I play mostly on sites where I get rakeback such as Cake or Absolute and just play a few tourneys on FT.