Hello,
Gnumeric uses XML too for its native format, so this filter
wouldn't be too hard to create.
But I can't do it because I can only do some QBasic (I promise I
will start to learn C++ within this week).
KSpread should really be able to import and export Gnumeric-files
because no one should say "free office-suits are incompatible".
The same is for the others KOffice-applications.

with regards,
Max Moritz Sievers