The primary data file requires one or possibly two changes to implement the memo pad:

Addition of a 2-byte or 4-byte binary link field. This field is used by INMEMO.SBR to link the primary data record to the corresponding comments which are stored in the auxiliary file. This field does not have to be manipulated in any way by the primary file maintenance program, except to supply it as an argument in the XCALL INMEMO... statement.

Removal of existing comments field. This is optional of course, but there doesn't seem to be any need for other comments if you have INMEMO.