Hi Pradip,
To upload bulk organization structure in OM we use LSMW or Custom ABAP Program. To upload organization structure through LSMW, you have to execute the transaction and follow 14 steps.
In the first step while creating recording the system will ask you to enter a transaction code and it is recommended to use "PP01"
To your next question, yes it is compulsory to implement OM in case if you are doing posting to finance and other necessary steps. In case of practice purpose it is not compulsory to configure OM you can always use "99999999" default position to move ahead.
Let me know if you still need any clarification.
Thanks & Regards,
Prasad
