An external character and gender neutral voice

The POSE function used with Storyline's characters isn't available for external characters.

The picture of an external character works like other inserted photos in Storyline. That means you have to separately insert the picture of each pose, or use the Replace Picture feature. 

As for voices: If you don't like any of the available voices for text-to-speech, you'll need to get your audio produced outside of Storyline, and then insert the MP3 files.