It's common for music to be written that incorporates existing themes / measures of music written by someone else in the past.  There is currently no way to mark on an MB Work that one of the credited composers did not participate in the creation of the MB Work in question.  This information is frequently available via interviews or common sense (i.e., the composer is deceased), and would offer a useful way to filter MB Works attached to a writer.  It's also frequently available in booklets themselves (e.g., "original music composed by" alongside a separate "music composed by" credit, or an explicit statement of "quotes music from" / "contains theme from")