That's kind of why there's the ability to convert the lines for export. For example, if you want different lines to be spoken with a different speaking animation you could do this:
1. Create your speech function: Character.SayStyle( string text, SpeechStyle style )
2. Include the style in your dialog lines, for example:
ego-eSpeechHappy: I'm happy!
ego-eSpeechSad: Well, not that happy.
3. Write the line conversion regular expressions: (these might be buggy)
Search pattern:
(?<name>[^:-]+)-(?<style>[^:]*):\s*(?<line>.+)
Replacement pattern:
c${name}.SayStyle("${line}", ${style});
Now, the lines will be converted automatically to:
cEgo.SayStyle("I'm happy!", eSpeechHappy);
cEgo.SayStyle("Well, not that happy.", eSpeechSad);
I don't think it's really worth the extra work to have the ability to define your own custom speaking functions just to support them in the preview, which is exactly why I added the conversion feature. Actually, I don't think it's documented in the help file but the conversion only works if the line format is: "[text]:[more text]".