<p>I use FB1.5.3 and need to eventually add some FK to the database.
It happens that if I try to do it while anyone else is connected to the DB I get an exception.</p>

<p>Is there a way to add an FK while the system is running?</p>

I believe this is a limitation on firebird 1.5 and earlier.
Try google for (firebird foreign key "object is in use").
hth