Advise whether to add a field to the Incident Adit log is possible? And how to do it?
Example: to track of changes to 'Service Type' and 'Status Reason' from HPD:Help Desk.
 

Yes, it is advised to enable audit on the fields per customer's requirement.

NOTE : The Status Reason (DB ID : 1000000881, DB Name : z1D_Status_Reason) field is a Display field and hence it is not possible to audit this field.

To enable the audit on status reason, use Status_Reason_Hidden (Database ID: 1000000150, DB Name: Status_Reason) which is set to Hidden on the form.
Woflows updates this field whenever there is change on the Status Reason field.

Also, you can enable audit on field Service Type (DB ID : 1000000099) field.

Steps :
1. Login to developer studio
2. Open form HPD:Help Desk
3. Click on fields out of view and look for field Status_Reason_Hidden (Database ID: 1000000150, DB Name: Status_Reason)
4. Open the properties of a field
5. Go to Attributes -> Audit Option -> Select AUDIT
6. Save the form
7. Flush/sync the Midtier cahce and test after re-logging into the server.
8. Make some changes on an Incident record for status reason field and check in the Incident Ticket -> View Audit Log option to and now you should see field gets audited.