Unable to add Key Mappings for Entity Associations on existing Aurena Custom Projection

I’m having an issue in Aurena when trying to modify a custom projection that I recently made. Aurena doesn’t seem to have the ability to add key mappings for entity associations when modifying an existing Projection in Aurena. You can add additional entities, and entity associations, but cannot specify the key mapping for that association. This means I cannot link child LUs to a parent LU for a custom page in the projection after it's made. Is there a current solution or workaround that addresses this? Is this a known issue?  I’m using IFS version 10 update 9.