This has two parts. The second one, having the extra dates sync with iCal/Calendar, I don't (yet) know how.
The first one, though, isn't so hard. Go to Addressbook/Contacts preferences. Under templates, click on "Add field" and select "dates."
Now you can customize the template to default with "Anniversary" or whatever other nametag you'd like to give the date field or fields.
I'd really like to hear how, if it's at all possible, even, to have those sync in iCal/Calendar.