You can download products and patches from you McAfee downloads page on a PC and move them to the offline ePO server to check in.
You can download VirusScan dat files specifically used for checking into ePO from the page below to your PC and them move them to the offline ePO server to check in and your clients will update from that.
It would be a pain doing this on a daily basis though, sometimes multiple update in one day. If the server is on the same network as a client with internet access you could automate it but if its on a fully disconnected/ seperate network it could be tricky.
As gR says, yes you can do all the above but the manual maintenance can be a bind as new Dat content is certainly posted daily.