Abacus is incredibly flexible and wherever you see a drop down menu, you can edit the default options or add new ones.
Click on 'Settings' and then on 'Data Lists'.
1) Click on the drop down box and you will see all the options where you can add or change options. Choose the one you need.
2) Click the green plus sign and fill in the name of the item you wish to add.
3) Click Save.