ImportSecurity.cmd is what you're looking for, it's documented in the Planing admin guide, and is easy to use. First run an export to get a baseline file that you can modify with your object security.
From the Planning admin guide:
The ImportSecurity utility loads access permissions for users or groups from a text file into Planning. (To add users or groups, see the Oracle Hyperion Enterprise Performance Management System User and Role Security Guide.) Importing access permissions overwrites existing access assignments only for imported members, data forms, data form folders, task lists, Calculation Manager business rules, and Calculation Manager business rule folders. All other existing access permissions remain intact. The SL_CLEARALL parameter clears all existing access permissions; you can use it with other parameters to replace existing access permissions. See also “Exporting Access Permissions” on page 60.
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