We can create custom fields in the Address sub Record by creating them as other record fields. But these custom fields will not show in the Standard Address field.
To display the custom field in the standard Address field, we need to customize the corresponding address form.
Navigate to Customization -> Forms ->Address Forms
Under the Fields tab, find the custom field created.
Copy the Template Field ID of the custom field and paste that to the Address Template field.
Please refer to the below screenshot
Save the form and repeat these steps for other Address forms also.
Then this custom field will show up in the Standard address field in the entity records