I must modify some Derby DB's structure, are you aware worthwhile (multiplataform) editor for carrying this out?

Also take a look at DbVisualizer

H2 Console - something from H2 database is another good a editor for Derby DB :)

Take a look at RazorSQL. It supports Derby and every one of the main dbs like Oracle, PostgreSQL, MySQL, MS SQL Server, etc. And in contrast to a few of the other tools, it really has very good support for a few of the less popular dbs like Derby, HSQLDB, H2, Firebird, SQLite, etc.