You cannot; the 2003 format is different from earlier versions.
You must create a 97-2002 PST file on Outlook 2003, then copy
(drag-and-drop) all data from the 2003 file into the new (old type) one.
This can be opened with Outlook 97~2002.
Are these .MSG files? If so, you must disable Unicode before saving them.
In OL 2003, cliock Tools>Options>Other>Advanced Options and uncheck the box
labaled "Use Unicode Message Format when saving messages".