It takes a fair bit of hand coding to make this happen, it is not a simple edit and is not supported.
You have to edit the CSSMenuWriter/intermediate.php file to dynamically create a spry accordion for each category
In the intermediate.php file, there are two loops, the first loop displays each category, use this loop to create the spry accordion. The second loop shows all the items in that category, use this loop within the collapsible frame.
If you are comfortable with the Spry Frame work and hand coding PHP, this should not be to difficult.
Just FYI, this is a feature we hope to offer in PowerStore V3