Can I make a backup of the Volume Plugin's settings?

Yes, you can manually copy or replace (with a backup) the configuration file for the Volume plugin.

The plugin's settings are stored in the following location:

SD 'root'/DirecTV_Remote/DirecTVaddonVolume.ini

Although it may not be necessary, it would probably be a good idea to either reboot the phone or force-stop the plugin in the task manager before trying to replace the existing configuration file with a backup.  This will ensure that the plugin does not inadvertently over-write the changes contained in the replacement file.