I don't want to spoil your day, but are you sure that GIMP will save to .lbm format?
I just tried and couldn't find it in the format list.
Or maybe you have something separate for the conversion? That would be interesting.
There's a freeware app called the "36-Image Converter," which Urban has mentioned, and which I have also attempted to use. The GIMP will not save to lbm, but the 36IC says it will convert to that format.