Yes. You can add custom fields to Click's entities. Follow the best practices outlined below:
Forms
If you wish to add your own custom fields to Click forms, such as the Email Send form, we strongly recommend that you add your own section to the form, and place your custom fields in the new section. This will help ensure that your customizations remain after you import an updated Click solution in the future.
CRM is responsible for merging your customizations with those contained in the Click solution. In some cases, CRM may remove your custom fields from a form and you would have to add them back after updating Click. You will not lose any data stored in the custom fields, they might just be removed from the form. Placing them in their own section on the form will help CRM to better merge customizations.
Views
If you wish to modify the views of Click's entities or add your own columns/fields to views, we recommend that you create your own views, rather than modifying the views provided in our solution. If you modify the views provided in our solution, they will revert back to our view layouts the next time you update Click.