By default you can not adjust the overrides for a nested reference.

Go to File/Settings/Configuration/Configuration Variables

Select Category - Levels

This can be changed by setting: MS_LEVEL_EDIT_NESTED_ATTACHMENT_LEVELS=1

Once ON, nested attachment levels editing will be enabled in the Level Manager as shown below.

This and a whole lot more can be found in the ReadMe file.

MS_Level_Edit_Nested_Attachment_Levels variable is Undefined in Workspace

MS_Level_Edit_Nested_Attachment_Levels variable is now set to 1