It isn't a supported option in the C170. If your appliance is out of support, you could always try to put more memory in the appliance at your own risk. I have not seen anyone do this - and not sure if the OS would actually recognize the altered memory configuration for that series hardware.
You probably would be better off in deploying a virtual C100v or C300v to off-set a current working C170 if you are seeing performance/processing issues. The virtual could be run in-conjunction with your existing hardware appliance.
http://www.cisco.com/c/en/us/products/collateral/security/email-security-appliance/data-sheet-c78-729751.html
