thanks for the answer
another USB serial convertor chip installed on the shield board with it's own USB plug is probably over my capabilities.
the a 'USB wall connector' is nothing more than a connector I have taken from a garbage printer, installed on the wall of my box to simply connect my USB cable on it. then I have connected 5V and gnd, and need now to connect com pins.
I have no space to plug something in the onboard USB jack because of my box. So I think I will solder these 2 wires, no the best but too late to change
thanks again