The Partner Object itself does not have the ability to have custom fields added. However, if the field can be populated as part of Account and/or Opportunity, it can be added as a field to be displayed in related lists. Here's how to select additional fields from account/opportunity object to show in Partner related lists:
Click on Setup – Customize – Account Layout – Related Lists
Go to Partner Related List –
Click on Settings icon. Select fields that needs to be displayed on Partner list and save changes.
Written by Prabha Krishnamurthy