The thing with the Z series is that it uses a proprietary SSD design, not a standard off the shelf 2.5 or even 1.8" SSD, so in short no, you can't, at least not quite as easily as you can with most.
The dual and quad SSD's that these support are actually all on the same pcb, they are a series of Nand Flash chips on one pcb that are configured in a raid array, think of it as having several separate devices on one board, you get the benefit of raid but without having to actually find the physical space for 2 or 4 separate drives, which a Sony Z series clearly could not accommodate.
There is however a way to use the SATA connection for the optical drive, with a bay adaptor to accommodate an after-market SSD though, I think there is Lenovo upgrade bay that will work with this with a little modification., but this will only work correctly if you are using a dual or triple raid Z series not one of the ones configured with quad stripped SSD's, as the Bios of the Z seems to have issues when what is effectively a 5th SSD is added to the mix, you can remove the raid array and use each device as a separate device though, but this requires a hacked BIOS.
I suggest you go and have a look at
http://forum.notebookreview.com/sony/ ...there is a ton of information about the Z over there and what you can and can't do with it in this regard and what you need to do it etc.
In a nutshell though, you can do this if you are willing to lose the optical drive and do some research and some fiddling around, there really won't be anything Sony will/can do for you other than changing say the dual 64GB SSD's (this is what the standard preconfigured model has), which is 2 x 64GB = 128GB on 1 pcb configured in a stripped array for a 192GB or 256GB device, which is still 1 pcb but the 192GB one is configured as 3 drives stripped 3 x 64GB and the 256GB device they offer which is 4 x 64GB stripped and appearing as a single 256GB device for your use.
Be careful though, the American market Z series use different SSD configurations to the European and Asian market machines, we actually get the better deal here for a change. An example of this is the 256GB US market machine is actually just dual raided devices, 2 x 128GB SSD's unlike the quad one we get and the 192GB device in the US is actually 1 x 128GB and 1 x 64GB devices ...again though all on 1 custom PCB.
