You can install Windows 10 unactivated as well.
Even if you have a license key, I would not enter it as part of an "internet-less install" as verification/activation will fail anyway due to having no way to reach Microsoft's servers. You can enter it, though, as in theory upon first connection to the internet Windows 10 should attempt verification and activation on its own. I just choose not to do it this way.
Once you have internet access, use Settings, Update & Security, Activation Pane to complete the activation process so you can see for yourself that the activation has completed successfully and be ready to call Microsoft if it doesn't.