It is possible to replace the odd with a 2.5" hdd caddy.
But from what I know the chipset in that laptop only have two sata3 ports and they're used in the internal hdd bays. So you really want to put a new ssd in one of the bays and move one of the hdds to the caddy. Since the odd sata port is only sata2 and that will be a limiting factor for a modern ssd speedwise.
I use one of these
caddys in my own laptop.
Asus ROG G55VW:
CPU: i7 3630QM
GFX: Geforce GTX660M with 2GB ram
RAM: 2x8Gb 2133MHz Kingston HyperX Impact (surprisingly the G55VW uses the 2133 X.M.P. profile)
Storage: RAID0 with 2x 256Gb Crucial SSDs (both msata, one in a sata adapter)
OS: Windows 10 x64