You can create your own additional fields (Example: "Invoice") in the database using the "Config > Setup > Custom Fields" ribbon bar option. The software will permit you to display custom fields (the ones created by you).

You can edit the additional fields in the database using the "Config > Setup > Custom Data" ribbon bar option. You can select the fields that should be modified so that the rest of the custom columns are not modified. The data corresponding to a group of domains also can be edited by highlighting the domains and then clicking the "Custom Data" button.

The lower panel in Watch My Domains ISP contains two views in a tabbed interface. The first one shows the whois data (raw data) that was retrieved from the whois server. The second one called details displays all the extracted data for the currently selected domain. You can manually edit the data in the details window.

If you edit a record using the Details View and then run a domain whois lookup your edited data may be lost (only the fields that are updated by WMD/ISP will be lost). To prevent this from happening, the software will automatically write-protect any domain that has been modified manually (you can change this behaviour from the "Application Settings" option).
Setting the "write-protect" will disable WMD from looking up that domain, so it will be better to do a lookup first and then use "Details View" to edit and lock.