Is there any way to turn on the built-in voice dictation in Yosemite with a voice command (just like we can do using the voice command "wake up" in Dragon NaturallySpeaking, or "Hey Siri" in Siri)?
Enable Dictation Commands
Before you can tell your Mac what you want it to do, you need to turn on Dictation Commands.
When you enable advanced commands, you can also create your own commands by clicking the Add Command (+) button. This lets you link a spoken phrase to an app, a menu item, a keyboard shortcut, or an Automator workflow.