Ok, but it means we need theologians who explain us how we have to understand a dogma, and in this particular case, that St. Agatho's words cannot be taken literally and, therefore, the Church can add to what was once declared.
Ladislaus answered you pretty clearly.
The Creed is not "a dogma." It is a profession of Faith in several dogmas.
Reciting the Nicene-Constantinopoliatan Creed as it was originally composed, minus the Filioque, is fine - many Eastern Catholic churches do this. But the dogma of the Filioque must still be believed, and so it was, by St. Athanasius, and most of the other Latin and
Greek Fathers who interpreted St. John xx:xxii for what it was - an unambiguously clear procession of the Holy Ghost from the Son.
Adding the Filioque was necessary in the Western Church, because Arianism persisted in the Latin West long after it had died out in the Greek East. It is merely explicitly stating a dogma in the Creed that was already expressed implicitly. It was done with the approval and permission of the Supreme Pontiff and that alone makes the addition licit, as the Pope is Supreme Legislator over the entire Church. The Filioque never became a point of contention until Caesaropapist schismatics like Photius and Michael Cerularius used it (and the Latin use of azymes and many other non-controversies) as an excuse to justify their schismatic actions.