To add a new field (column) to an existing table you can use an ALTER TABLE statement like this:
String connectionString =
"jdbc:odbc:Driver={Microsoft Access Driver (*.mdb, *.accdb)};" +
"DBQ=C:\\Users\\Public\\Database1.accdb;";
Connection con = DriverManager.getConnection(connectionString);
Statement st = con.createStatement();
st.execute("ALTER TABLE Customers ADD COLUMN Address TEXT(255)");
For more information on ALTER TABLE see