How to get a participant's line manager in approval flow?

Summary:

Need to get approval from participant's line manager for a performance document

Content (required):

Dear Experts:

We got following Business scinario:

Worker XX has Line manger AA and Matrix manager BB;

AA has Line manager MM;

BB has Line manager NN

In a performance document of XX, AA gave rating as line manager and BB gave the feedback as participant. After AA sumbit the document, a Parallel approval flow should processed as below:

MM and NN should received the approval request at same time. Both of them approve the appraisal means approved , either of them reject means fail;

We can see the Edit Participant functionality in Transaction console, but can' get NN in this UI.