MOSS to GRASS is difficult. So much so that it has not et been done.
The principal problem is that in MOSS each vector polygon is complete,
and overlapping polygon boundaries do not have to match exactly.
When exported, this created slivers, overlaps, etc. Best approach is
to learn to compile and use LTPlus in 4.1 src.related release. This
will import MOSS, allow cleanup, and export to GRASS.
Jim Hinthorne
GIS Lab, CWU