Ah, I see. XWE can actually open "single entry" files. If you open any file with one of these extensions: '.ART', '.PPM', '.CPS', '.PAL', '.WAV', '.VOC', '.BMP', '.PCX', '.JPG', '.TGA', '.ACT', '.BYT', '.DTX', '.NWX', '.PNG', then XWE will load that one entry. However, it goes into read-only mode, this is mainly done for ripping from all those supported games.
I suppose I could add .LMP to the list, so that it would open single lump entries, and enable actual editing.