Yes, You can but it will create redundancy.
Suppose,In employee table primary key or candidate key is EmpID.
So From EmpID we can derive all the other attribute like EmpID--->Ename,gender,city etc.
Now, If we put SupID in main Employee table then we can't say EmpID-->SupID. So Candiadate key will become EmpID,SupID.
Because Of above candidate key EmpID----->Ename,gender,city etc. become partial dependency so whole relation will come in 1NF.
Second reason is , Here many to many partial relation so employee may have supervisor or may not be. If employee doesn't have supervisor so SupID become NULL and It it is not possible because SupID is primary key of employee table and Primary key doesn't contain NULL value.