Yes you can do this.
Create the parameter with for example, default values of "Item" and "Customer". Then write a formula field based on the parameter value:
If {?Parm} = "Customer" then {Customer} else
if {?Parm} = "Item" then {Item} else .....
Put as many levels in here as you would like, and put the defauls values for the choices in the parameter field.
Then group on this field, the TopN report doesn't care what you put in the formula, it will just return the Top N of whatever you want.
Software Support for Macola, Crystal Reports and Goldmine
dgilsdorf@mchsi.com