Each output channel corresponds to a bit in a PC I/O port, making the PCL-734 very easy to program.

So basically you would have to control Com port registers (where the serial IO card would be mapped) and write Hex numbers to the internal addresses of the PC from WinCC Flexible.
There was a recent discussionthat addressed this issue, and it looks promising:

Write data to the Serial Port in VBScript

Hope this helps,
Daniel Chartier