There are a lot of devices out there to convert a serial device to USB for interfacing to a computer, but I need the other way around. I have a USB touch screen device that I need to interface with a PC that only has RS232 serial inputs. The touch screen, which itself appears as a USB mouse, must be converted to appear to the PC as a serial mouse.
Its not clear to me from the description - is the "USB Serial Light Adapter" capable of this conversion? If so, when might they next be available?
I suspect it will not be so simple. AFAIK ...
USB devices, such as your touch screen, rely on a complex communication with the PC to identify themselves. That concept did not exist before USB was "invented".
Another problem is that the touch screen expects to connect to a USB Host - which is what has the appropriate driver software to understand it.
A USB-Serial connector is only intended to create the equivalent of a serial port on a PC that has USB host ports. The USB Serial connector itself expects to connect to a USB Host - it is not a standalone system.
Some Arduinos offer full control of the USB port (Leonardo...), perhaps you'll find a library that implements an USB host on such a board.
It is possible to re-program the USB chip on all the Arduinos. But I reckon if the OP knew how to do that he would not be posting a question here.
This is only an option for people with the necessary experience of the complexities of the USB system. Also, I don't know if all of the USB Host code for a specific device can be accommodated in the USB-Serial chip. Perhaps some of it must exist in a driver on the PC?