To add a new pension scheme under additional self defined Pension settings is under Pension button in the Tasking zone. Please follow the below Road Map:

Road Map - To add a new pension scheme under additional self defined Pension settings
Pension -> Auto Enrolment Pension(AE)Setup -> Select the Pension Provider -> Additional AE Pension settings Link -> Click the Add New Pension Scheme button -> Click the Save button.
('Pension' is the button positioned in the tasking zone menu at the top. It is the third button from the Right)