Threading a threadless fork is a difficult, costly process and a lot of shops won't even do it. Extending the threads on an already threaded fork can be done much easier.
Just buy a new fork. Here, I'll do the work for you:
http://cgi.ebay.com/TANGE-ROAD-FORK-...item2a0e0cc51a
http://cgi.ebay.com/New-CR-MO-700c-R...item4cf33c359f
http://cgi.ebay.com/straight-bladed-...item3f00de119e