You can create any number of custom fields for expenses.

To create an expense  custom field:

1. Log in as Administrator (or a user who has the ‘Manage Customers’ permission). If you are already logged in, skip this step.
2. Select Custom fields on the User menu.
3. Select Expenses under 'Custom fields for'.
4. Fill out the form.
5. Click 'Add'.
6. Repeat steps 4 to 5 to add more custom fields.