You can't with 7.1. What you can do though is right click on the assistant and select 'Open Front Panel'. This will convert the assistant to a normal subVI and then you can edit it to add the VISA Configure Serial Port function and create controls for the inputs.
Using Instrument I/O Assistant you cannot change the serial port parameters in any version of LabVIEW. In all version you will need to use the VISA Configure Serial Port to do this.
Regards Krista S. Applications Engineering National Instruments