No, this is not supported out of the box in Dynamics 365 Sales.
The Activity Type field is a system-defined option set that represents the underlying activity tables (Email, Phone Call, Task, Appointment, Letter, Fax, etc.). It is not a configurable choice field, so:
❌ You cannot remove the default activity types.
❌ You cannot add custom activity types to the Activity Type field.
❌ The option set cannot be customized through the solution explorer or Power Apps.
Posibility:
Create a custom Choice (Option Set) field
Add a custom field such as Activity Category or Business Activity Type.
Populate it with your own values (e.g., Customer Visit, Follow-up, Sales Call, Support Call).
This is the recommended and supported approach.
Create a custom Activity table
If you need a completely new activity type (e.g., "Site Inspection" or "Product Demo"), create a custom activity table by enabling the Activity option when creating the table.
Your custom activity will behave like other activities and can participate in timelines, activities, and related features.
Note that this does not add a new value to the system Activity Type option set; it creates a new activity table.
