No problem. I had some customers ask about adding a ssd to their MacBook Pros and making a fusion drive from two different hard drives, rather than a hybrid. I wanted to try it for myself and see if it would make any difference in performance.
I added a refurbished SSD where my HDD was and put the HDD in the optical drive bracket. There were instructions on how to enable this in Terminal on about.com. Performance seems to have increased some, but not as much as a larger SSD by itself.
I don't know if this would be worth anyones time, or the amount of money they would spend in the end. Basically I had to backup everything, because my regular hdd was going to be erased in the process. Then I had to boot from an external drive to enable the fusion of the two internal drives. After that I had to wait as time machine loaded everything back from the external drive. It took about 4 hours total. For what I would have to charge a customer, They could probably just buy a large enough SSD and use an external for anything extra, and they could still keep their ODD. If storage was really an issue, you could always just add the old HDD to the bracket and have two separate internal drives. The only advantage to the fusion setting is that it keeps it simple for the user, and you can use a much smaller (cheaper) SSD only for the files you use the most