Yes you can add in a SSD into your system, but you'll need to give up your optical drive. Or, you could just replace your current hard drive for a hybrid drive.
A hybrid drive will offer the best performance in your system as the optical drive in your system is a PATA drive.
I've changed my mind on Fusion drives in general, after dealing with some problems with there use. The best approach is to setup the drives independently and manage the SSD as the boot & app disk and the HD as the data drive and a second copy of anything on the SSD. While that takes more upfront work it is safer long term.