I agree with Pim. I wasn't really clear about the HKCU\Software\Policies registry key.
You can add the excel15.admx to UEM's ADMX-based settings using "Manage Templates". After that create a new ADMX-based Settings and click "Select Categories".
Choose "Miscellaneous" (see picture).
Click "Edit policies".
Enable "List of managed add-ins" and click "Show" to add "OneStop.ReportDesigner" with a value of 1
Using the "Conditions" tab you can choose which users/groups receive this policy setting.
This should result in the GPO for disabling the add-ins being applied to all users and for those users that need the managed add-in for OneStop.ReportDesigner an exception is made by the ADMX-based settings.
You can also consider moving the other user based GPO settings to UEM (ADMX-based settings) like Pim already suggested.