I want to use my own or custom voice command for a Google Glass app using the Mirror API.
Is this possible? If so, how can I do this?
Yes, although you need to request them from Google. See https://developers.google.com/glass/develop/mirror/contacts#declaring_voice_menu_commands for more information about voice commands with Mirror. That page refers you to https://developers.google.com/glass/distribute/voice-form to apply for a voice command.