In case you want to add specific and the same additional field to a lot of entries, you can use templates for this. With templates, you can add a pre-set number of fields to the entry, with the names and default values as you have specified earlier.
To manage template, open the File menu and select "Organize Templates". There you can add new, edit and delete current templates. When adding or editing template, you can name it and specify which fields it should have. As you can see, you can also change the order of fields with up and down toolbar buttons. Additionally, you can set the option "Auto load" for a template, indicating that you want to add fields and values from this template automatically to every entry that is added in the program.
Now, once you have a template or some of them, you can start loading them to your entries. When you open entry window (add new entry or edit some current entry), then on the "Additional fields" panel you should see the "Templates" button. When you click it, the menu appears from where you can select the template to load to the current entry. Once selected, all the fields and default values are added to your entry. If your entry already contains additional fields, the program will ask if you would like to remove old values or keep them.