Hi,
I am not able to understand what is enabled and what is disabled in your case. But what I know is that your query can be easily handled through OPSU settings. You have given the screen shot of CN22 with Ext. Processing tab. Now, if you want to auto tick the Service field in that tab. Then you just need to go to OPSU and then to PS02 activity type. Double click on that and tick the service field over there. Its Simple..!! What is the confusion then ?
But what I understood is that your service field is disabled in activity type which is something that I don't think it can be possible. But if it is happening then why don't you use the option of "Copy As" and make new Z activity type and use it instead. Moreover, if you feel that you need to make the changes in entire projects/networks then use CNMASS tcode.
Regards,
Amit