If it's a session parameter that's used by the SqlDataSource, then you can set the value in the session, e.g in Page_Load():
Session["thisCategoryID"] = theCategoryId;
(am I misunderstanding the question?)
Ok, update:
I think you can create an event handler for the SqlDataSource.OnSelecting event. In that handler, you can access the Parameters collection of the datasource and can add another Parameter to it. I currently cannot test the following code, so it might not be fully correct, but I hope you see the idea:
SqlDataSource1_OnSelecting(SqlDataSourceSelectingEventArgs args)
{
var param = new Parameter("@thisCatagoryID");
param.DefaultValue = Session["SelectedCID"];
SqlDataSource1.SelectParameters.Add(param);
}
Alternatively, you can set the parameter declaratively in the markup, e.g:
<asp:SqlDataSource ...>
<SelectParameters>
<asp:SessionParameter Name="thisCategoryID" SessionField="SelectedCID"
DefaultValue="0" />
...
</SelectParameters>
</asp:SqlDataSource>
