A quick (and crude) way would be to pre-fill the field when you go to add a new product. Then, providing you don't edit the Extended Desc field, you'll submit the new product with the filename included.
Open shopa_addproduct.asp and scroll about a quarter of the way down (mine's on line 694) until you get to the line that says:
pcreaterow getlang("langproductextended"),"strcdescurl",strcdescurl
Copy the line and paste it again underneath, comment the first line out with a single apostrophe (as a backup incase you want to go back to how it was) and on the second line change it to read:
pcreaterow getlang("langproductextended"),"strcdescurl","shopexd.asp"
The difference being the file name for your extended description instead of strcdescurl. Make sure the file name is in double quotes.
My finished changes look like this:
' pcreaterow getlang("langproductextended"),"strcdescurl",strcdescurl
pcreaterow getlang("langproductextended"),"strcdescurl","shopexd.asp"
Save it and give it a whirl.