Hi Garima,
The question was already answered by you
I understand the same can be achieved by creating Organizational Assignment at the time Hiring / Promotion / Transfer, and this is applicable for new joinees only.
Once the relationship is created you can't edit the Percentage field which will be hidden and even if you check at data base level it is a display only field. Hence we can understand that It can only be possible at the time of new record creation rather in change mode.
Regards,
Srikanth Naidu. Akula.