Mago comes with many of the most commonly used fields already built in, but you can edit the fields in your records, and even add new ones using the system field editor.
Click on the profile menu and select "Edit system labels" from the drop-down list
You will be presented with a list of all the system labels and a text box you can use to input your preferred value.
You will notice that under the company, contact, and gig categories, there are custom fields you can use to add additional fields to your records that aren't represented already. Feel free to use those as you see fit.
Note that if you change a field name that relates to a merge token in your templates, the merge token will not change but it will still pull in the data and the field in your record will be labeled correctly. The merge token doesn't change in your templates because that would disrupt the merge process when you go to merge your data with your templates.