I asked the same thing (on Creative Cow), and the answer at the moment is that you can not add your own, but you can apparently modify the existing Lower Third templates, which resides in \Program Files\VASST\Common if I recall.
> well if you're accepting suggestions for improvements for the next version....
We are always open to suggestions on how we can improve our tools. ;-)
> You should add a text input layer to the lower third dialog so it will automatically create and layer a pro-titler object on the track above so you can finish the lower third in one shot
I like that idea and have added it to the "future" feature list.
I guess that would probably take a bit more coding to do than 'just' the user defined templates, however having the LT add a second track with a matching Pro Titler (or regular text) event would be nice.
How to define this text event, for the Lower Thirds that need it, is the question then. If it is possible for the tool to look into the veg files with the graphics, it may be possible for it to copy the tracks (or the events on them) that are marked as 'muted', video as well as perhaps audio, the title event could then reside on one of those.
It is just an idea, I just don't know enough about Vegas scripting to say if this is even remotely possible.