Hi, Cliff -
The normal way to access a stored procedure data source in a DataWindow is not available in a Crosstab. If you do not want to recreate the Crosstab, you can attempt to (carefully) edit the DW source and add the new argument's definition in the "arguments=(...)" clause (it should be just before the text, column, compute, etc. DWObject definitions). This limitation is due to the unique processing performed in a Crosstab-style DW.
If you edit the DW source manually, I strongly suggest you make a copy of the DW and experiment on the copy.
Good luck and best regards, John