Answer
You can bulk-copy the custom field definitions at the metadata level by inserting rows into CUSTOM_FIELD_ATTRIBUTES_TAB, selecting from the existing LU's rows (filtered to CUSTOM_FIELD_TYPE='PERSISTENT_FIELD' for a first pass) and rewriting the LU column to your new LU's name, along with updating any LU-name references embedded in the expression column via a string replace. Persistent fields need to be copied first; if you also want to copy read-only/computed custom fields, do those afterward in dependency order (fields that only reference persistent fields before fields that reference other read-only fields on the same LU). The new LU record itself still needs to be created normally first — this technique only accelerates duplicating its custom field definitions. Since this works directly against IFS's internal metadata table rather than through supported tooling, test it thoroughly in a non-production environment before relying on it.