Diana:
I looked at the code for the Shutter Rectangular 14 object, and the distance between louvers is the same as the louvre depth, so there is no individual adjustment for this distance. Since the same distance is used for both in the code, it is not a simple task to add another parameter for the distance between louvers without rewriting other parts of the code. If all the sunshades you are using are the same, and it is important that they exactly show as per the manufacturer's details, then you would need to create your own library part from the ArchiCAD tools, following Matthew's advice or perhaps a slab with a bunch of holes.
David