Problem solved! I eventually found the answer to my own problem. The dictionary file that imported fine had lines ending with both a carriage return and a line feed. The file that wouldn't import had lines ending only in a linefeed. When I turned on hidden characters in Notepad++, it was clear why one file was rejected.
I note, however, that the added.txt FrameMaker dictionary file that results ends in linefeeds only. FM apparently needs the CR to import the dictionary file, but strips it out when it writes the dictionary file itself.
