Answer
It's not possible to add more User_Def columns to an existing standard metadata table once its predefined slots are all used. The correct approach is to create a brand-new custom table and link it back to the main table via a properly defined Relation Definition, being careful to select the correct relation type for that specific link — this is the same approach a related implementation already used successfully (creating a separate custom table, with its own User_Def-style fields, connected back to the main table this way) rather than trying to add columns directly to the existing table.