Hi Adam,
The official answer is that there is no way to do this. Something that we can look into providing in a future release though. The unofficial answer is that if you overwrite the actor shapes library that gets installed with CaseComplete (UCActorsLibrary.nlx in the CaseComplete ShapeLibrary folder under program files) and replace it with one that includes your custom shapes, CaseComplete *should* pick them up. Standard caveats apply - this is unsupported so no guarantees. In addition, writing to the program files folder is discouraged in general and if you're running Vista or Win 7 you'll have to get past the UAC warnings.
Doug