properties on fo namespace, like word-spacing

I am a bit surprised why the fo:word-spacing property as defined in XSL[1] is 
not mentioned in the ODF spec.
Is this an oversight? Are odf-writers allowed to just use all 'fo' namespaced 
properties that are defined upstream? Or maybe we decide to not support this 
one? (can't find any mention in the archives, though).

If its an oversight, how hard would it be to get it into ODF?

ps. Apologies if I'm asking obvious questions, I have not had time for any 
ODF-TC work for several months now :/

1) http://www.w3.org/TR/xsl/#word-spacing
-- 
Thomas Zander