My internet access is through a company system with a firewall, and thus my AppleTV is unable to connect. Is there any way to upgrade the ATV operating system without internet connection (my MacBook connects without problem)?
There's some pretty elaborate ways to do this that involve patchsticking the AppleTV and/or removing the HDD and connecting it to your Mac to upgrade but a simple solution would be to just take it with you to a place where you'd get WiFi and just do the upgrade there.
It's not like upgrades are available often so taking it to a WiFi hotspot for the occasional upgrade isn't much of a hassle.