In its current format you can’t change the system parameter. So in order to get what you want out of it, you need to cheat it.
Go to Settings > Family Category and Parameters, select Generic Models and click OK.
Now go back to the Family Type Dialogue box, and you will be able to select the system parameter and hit modify. Change it to an instance parameter and select OK.
Now change the family back to a Casework family, and bingo.