First, welcome to the forum, Shaun!
One way I'd recommend to approach a custom sidelight like this: create a custom panel for the sidelight. You get an option of a custom door panel and a custom sidelight panel.
https://www.youtube.com/results?search_query=custom+door+archicad
Note that once you have a basic understanding of how to script a parametric object (one that retains certain dimensions when overall size changes, instead of stretching like a rubber band), you can actually script a very flexible door or custom panel, with various automatic parameters. You can then use one "smart" custom panel regardless of door or sidelight size. Example: A sidelight panel that automatically increases the amount of glass divisions once the sidelight gets bigger.