What kind of PLC and touch screen are you using?  I have done this with an AB SLC via the comm port and device net (uggg).  You can manipulate the string (and will probably have to) once it is in the PLC.  If you have a simple fill in the blank situation, you may want to just bring it into the touchscreen and have it fill out a section of the form.  I generally prefer the PLC for most applications because it gives me more control over all my data.

The easiest solution I can think of is to replace the DirectTouch screen with a Red Lion G3.  You have 3 available com ports to bring in bar code readers and talk to the PLC.  The bad side is you would have to re-write the touchscreen program.

The 5/03 has a D-sub 9-pin serial port.  If you are not using it to communicate with the touchscreen, you could use that port on the PLC to bring in the bar code.  There are some ASCII conversion instructions in Logix500.  The only catch is you will have to change the default com port settings in the PLC to the bar codes' settings, which is probably 9600 baud, no stop-bits, 1 parity bit, and you will have to change the default protocol from DF1 to ASCII.  Once you do this you won't be able to communicate through the port again with RSLinx.  You would have to go through the DH485 port with a PIC module.

One other option is to get a SLC500 co-processor module.  I think the part # is 1746-BAS.  They are very pricey though.