Is it possible to a HTTP Download Proxy added to the firmware. I'd like to add a USB stick to the 2925 and, this this for HTTP Proxy at the router level so I've not downlaoding the same thing repeatly.

Eg, I have 4 MAC's, I get a update to Microsoft office about 1 a week, Outlook is 1.2Gb per machine. I'd like to do first as proper downlaod and second, third and forth as downlaod from the cache.

Something like a squid proxy would be fine.
Would this be possible ?

A proxy would likely not be able to cache this properly anyway. These days caching proxies struggle to cache many things because content changes so rapidly.

Just my opinion anyway