How to do this in Change Log Entry table

Hello, this is a question for that people that use the "Change Log Entry" table. There is a field called "Primary Key"; here appers the keys from the table that you want to controller. Example; if you want to log the entries from Quantity field from Purchase Line table, in Primary key appers Type, Document Nº and Line Nº. because are the primary keys form Purchase Line table, but, what I have to do to appear also Nº. field?, I don't want to put this field on the primary key from Purchase Line table. Any idea please?.
Thanks in advance.