My project has a number of test PCs which are 100% off-net. We use a USB drive to transfer files too and from these PCs. Some development happens during the tests on those PCs.
You can also keep a clone of the repository on a USB drive. 'push' from the off-net PC to the USB drive, then 'push' from the uSB drive to the central repository.
It might be better to have the USB drive repository with its own working directory (also on the USB drive). Then 'pull' to the usb drive from the off-net PC, and 'push' from the USB drive to the central repository. When that push happens, you can do merging, if necessary, on the USB drive.