If I am remembering correctly, aren;t the Import COnfig fields actually a series of potential fields? You can mix literal, scanned, and calculated values and concantenate them all together. So, you should be able to make a literal portion of the field say "WEB" and then add another part to that field with the scanned in PO value.
Let me know if you can see what I mean -- what import config screen are you using? 6.11 or before, or 6.12 or 7.0? I do not know what the import config screens look like for 6.12 and up...