You can! In Yahoo! Address Book, these are called Custom Fields. You can add them just below the "Notes" section at the end of the "Edit Contact" page.

1. With your Address Book open, click Edit beside any name.
2. Scroll to the bottom of the page, and click Add a Custom Field.
3. Click the down arrow to open a pull-down menu, and click [New Field].
4. The "Add a New Field" box opens. Type the new field name, and click OK.
5. Now that you've given the new field a name, click in the box to the right of it to fill in whatever information you'd like this new field to contain. For example, if you created a new field called "Position," maybe for Julius you want to put "Forward". :o)
6. Click the Save button.