If You got Great Sound from GarageBand - Then use this to do the VoiceOver. Make this as the last action in Your Video editing process.
I use a Stereo SONY Mic and my miniDV tape Camera - and recorde the audio with this when playing the movie on My Mac so I get it precisely timed. Yes I'm lazy.
Yours Bengt W
You need to select the Blue Yeti as your input device.
First go to System Preferences/Sound and click on Input Device. Make sure your Blue Yeti is in the list and selected. Make sure the red light is on on your Blue Yeti. After a period of unuse, it may turn itself off.
Then, in iMovie, start your Voiceover session by pressing the Microphone on the middle toolbar. You will get a screen like this where you can choose the Blue Yeti.
Then follow the instructions here.
After you have recorded your voiceover, you can select the Voiceover Track and make additional adjustments in the Audio Inspector. For example you can use the Graphic Equalizer.
Having said all that, Garageband is a great tool if you need even more adjustments. You can export your movie to the Media Browser and do this in GarageBand, or, you can record a voice track in Garageband, render it as AIFF, and use it in IMovie.