There are two servers around — caching server, and software update server — and I'll discuss both here, as I'm not sure which you're looking to provide.
The caching server requires a connection back to Apple, and the public IP addresses involved with the caching server and the clients must match. This connection and this address matching is a prerequisite for the caching server. In short, caching server won't work off-line.
The software update server can be configured to acquire its updates from another internet-connected software update server running within your network; to cascade updates. This was discussed in the old Snow Leopard 10.6 OS X Server administration manuals and in a support note for newer releases, and hasn't particularly changed over the years.
If you're seeking to transfer OS X and OS X Server updates to a remote or isolated or limited-bandwidth network using software update server as I'd suspect, then you're either going to need an intermittent connection to the network-connected software update server, or you'll have to haul what you're calling the offline server around manually. I'd not recommend trying to haul around the update files underneath the software update server, as that may well and variously has led to corruptions, when folks have tried it.