Sony Ericsson Xperia X10 Mini/pro :: Add A Category To Contact?

I've recently bought this phone and imported all my contacts from my Gmail address book. Every contact has at least one assigned category, and some have multiple categories. I can filter my contacts on the X10 Mini Pro, which greatly assists in finding people, but I can't see any way to add a category tag to a new contact. I've looked at all the manuals I can find, and don't see this point addressed. Lots of information about adding photos and Timescape, but nothing useful.