If the position of the Catholic Church is true, then the notion of sola scriptura is false. There is then no problem with the Church officially defining a doctrine which is not explicitly in Scripture, so long as it is not in contradiction to Scripture.
The Catholic Church was commissioned by Christ to teach all nations and to teach them infallibly—guided, as he promised, by the Holy Spirit until the end of the world (John 14:26, 16:13). The mere fact that the Church teaches that something is definitely true is a guarantee that it is true (cf. Matt. 28:18-20, Luke 10:16, 1 Tim. 3:15).