To assign Supervisor to a user
Goto
tcode - pa20

get both user and supervisor "organization assgment"
user  aka staff
62311887 <-example the user's org assg

supervisor id
62423005 <-example the user's supervisor

tcode - pp01

insert
Plan version  - Current plan
Object type - Position
Object ID - 62311887 <<enter the user id we just obtain like the example above>>


Select and maintain (pencil with spec -edit mode)
Select "Relationship" and edit (pencil - edit infotype)

look for the first record or line with

[[Relationship type/relationship     A  002  Reports (line) ]]

then enter the field "ID of related obJECT" with the supervisor org assigment"

62423005

Click save.