I just figured this out a few days ago, as I had the same need, but you should check it first.

PROCEDURE ShortPathName
PARAMETERS pLongPath

* Uses WINAPI to get DOS8.3 type name

DECLARE INTEGER GetShortPathName IN kernel32;
STRING @ lpszLongPath,;
STRING @ lpszShortPath,;
INTEGER cchBuffer

LOCAL lclBuffer, lLen

lclBuffer=SPACE(250)+CHR(0)
lLen=GetShortPathname(pLongPath+CHR(0), ;
@lclBuffer,250)

RETURN left(lclBuffer,lLen)