Hi Shaila
Hard to say. Note 44410 explains in detail how to extend the backend (and is also quoted in SAP's extensibility guide in http://scn.sap.com/docs/DOC-7858#section2 ).
You'll probably want to create your own Field Selection Group (some numbers are reserved for customers, I believe).
Mandatory/optional: Up to you. Optional will always do fine.
Maintenance status: Up to you - for MARA fields, K (or all statuses) will do fine.
Regards, Ingo