How to include a new Z Partner Function in Sales order header drop down lis

Hello SAP experts,

We have come up with a new requirement to include a custom Partner Function-ZI in dropdown of SO header-Partners field.

Current scenario- Presently we are maintaining this partner function ( Say ZI- Institute ) in customer master data-Sales area tab.There could be more than one partners under this PF in master data. While entering sales Order, all partners associated are displayed in Header- partners tab, where only SP, SH, BP, PY partner functions are displayed.

Requirement- 1. Is to provide Drop down of this ZI-Institute partner function (which I did not find presently) in SO header-partners field and user should be able to provide one of partner function as available in Customer master ( as there are more than one partners that can be set in Customer master data but user need to be able to provide ONE of this ZI partner fucntion in DROPDOWN of PF tab in SO header ).

2. In addition, How can I link this new partner function in related sales reports against that particulat order and invoice?

Kindly suggest your valuable inputs as what should be the procedure to get this.

Many thanks in advance.