>
>Can I set the parameter value myself in the code?

I'm not sure - maybe through an evaluated tag? ex. < %= this.SiteID % >

Since we're talking about collections, you could also probably do this by accessing them directly:

Parameter selectParm = (Parameter)this.grdName.SelectParameters["NameOfParameter"];
selectParm.Type = TypeCode.Int32;
selectParm.DefaultValue = this.SiteID.ToString();

I haven't tested the above, so YMMV.