Been looking at the possibilities of chaning series to categories in Stanza. It turns out it cannot be done at the XML catalog level, but needs to be stored in the ePub book as metadta during conversion. Therefore for this to happen it would have to be something that was put into the ePub conversion code - and would probably need to be made optional for those who did not want it.
If you really want this as a feature, I suggest that you raise it as a ticket for Kovid to look at. I am personally less concerned as I can always look at the online catalog information.