Current State:

Currently, Assets Data Manager (ADM) does not natively support merging multiple rows from a data source 

• If the data is concatenated into a single field to fit one row, it often hits the 255-character limit, causing data truncation.

• If the data is kept as multiple rows (one row per user/group combination), the import process treats each subsequent row as an update that overwrites the previous value rather than appending it to a multi-cardinality attribute.

Proposed Future State:

Introduce a native "Merge" or "Append" logic within Assets Data Manager that allows multiple source rows sharing a common identifier to aggregate values into a multi-value attribute (Object Reference or Select list) without overwriting existing data or requiring external pre-processing.