Hi, petewinter. You might want to invest in a little time and assistance from a qualified DBA. A table maintenance script will enable you to change the contents of DB tables. It is usually a form and action-script pair that puts table data on your browser screen and lets you modify the table data.
I would not envision that this would create a new field -- a new field might not be necessary at all if the tables have unique keys. A many-to-many relationship between tables usually involves a "pivot table" that associates the keys of one table with the keys of the other table. In other words, it provides a linkage -- look up a key or keys in one table and the keys will point you to all of the associated tables through the paired entries in the pivot table.
HTH, ~Ray