Ah, yes. I looked at the spec sheet again and on page 3 it seems pretty clear that it supports a second HDD or SSD. Not sure how I missed that on the first read, but it clear that it supports two drives and an mSATA drive.
Open the case when you have time and see if it already has a bracket (looks like a small cage) in that location. You will be able to fit a standard 2.5 inch 9.5mm drive in the extra space according to the specs, so pretty much any current SSD will fit fine and work. You best bet would be to install the OS and key programs on the SSD and use the HDD for storage.
Additionally, it does support an mSATA drive so you always have that option too.