Could "The human soul is created at the moment of conception." be defined as dogma?

For something to be declared a Dogma, two conditions are required:

1) the statement is within the scope of Dogma. This is, it refers to faith or morals.

2) the statement does not contradict other Dogmas (because the Magisterium cannot contradict itself, as it is infallible on matters of faith and morals).

Regarding 1), it seems clear that the creation of the soul is about faith. Footnote 19 of the Declaration on Procured Abortion (1974), by the Congregation of the Doctrine of the Faith, discussing the views of the timing of the infusion of the soul, states:

It is not within the competence of science to decide between these two views, because the existence of an immortal soul is not a question in its field. It is a philosophical problem from which our moral affirmation remains independent . . . supposing a later animation, there is still nothing less than a human life, preparing for and calling for a soul in which the nature received from parents is completed. One finds here two important points: the question of when the soul is infused is not one that can be decided by any empirical means, and even if the soul were to be infused at some later point in embryological development, the zygote that is present at fertilization is surely a human life. As such it deserves the same respect as is due to any other human being.

Thus, in the most crude terms, the issue of the creation of the soul is not within the real of science, and therefore it belongs to the real of faith.

Regarding 2), the Church has not infallibly pronounced on the issue and there is no other infallible teaching related to this, beyond that (i) the soul has no pre-existence (Origen's related heresy; Enchiridion symbolorum, 403, 456) and that (ii) the soul is immortal (doesn't die with the body).

A comprehensive list of official teachings pertaining the soul can be found in the New Catholic Encyclopedia (second edition, 2003). I quote in extenso:

Solemn Magisterium. Interpreting officially the witness of the Scriptures and tradition concerning the human soul, the teaching authority of the Church solemnly affirms that the Triune God is the creator of the human creature ‘‘constituted, as it were, alike of the spirit and the body’’ (Lateran Council IV in 1215: ibid. 800; cf. Vatican Council I: ibid. 3002); ‘‘that man has one rational and intellectual soul’’ (Council of Constantinople IV in 870: ibid. 657; there the doctrine of the two souls in man was condemned as heretical); ‘‘that the rational or intellective soul is the form of the human body in itself and essentially’’ (Council of Vienne in 1312 against Peter John Olivi: ibid. 902, cf. 900, 1440); and that the human soul is ‘‘immortal and multiple according to the multitude of bodies into which it is infused, multiplied, and to be multiplied . . .’’ (Lateran Council V in 1513 against some humanistic Aristotelians who renewed the Averroistic monopsychism: ibid. 1440).

Ordinary Magisterium. Man is so substantially one, according to the ordinary teaching authority of the Church, that his unity (which is not accidental) has ontological priority before the real and irreducible plurality of his being. He is one in origin, being, and final destiny (Enchiridion symbolorum 502, 2828, 3005, 3221–22, 3224). Therefore each consideration of a part or one aspect of man implies repercussions concerning all parts and aspects. Any division of man is always inadequate, because as a microcosm (ibid. 3771) he must be considered as a whole. However, since there is an essential difference between matter and SPIRIT (ibid. 3891; cf. 3022–24), there is a real plurality of realities in man which are irreducible to each other. Thus the spiritual soul is not an emanation or a part of man’s matter or body (ibid. 3022, 3220–21, 3896), and it is equally true that the matter cannot be deduced from or reduced to the finite human spiritual soul. Both need a special creative act of God in order to exist, because they are ontologically different (ibid. 360, 3896). Thus man possesses the vital (ibid. 2833) and constitutive principle of his being, i.e., one spiritual, simple, and substantial soul (ibid. 791, 801, 900, 1440), which despite the substantial unity of being in man, is in its being and meaning essentially different and independent of matter (ibid. 1007, 3002, 3022, 3220–24, 3896), and immortal (ibid. 1440). Since the soul is spiritual in itself, man is not composed of three different realities, i.e., body, soul, and spirit, but is a substantial unity in body and spiritual soul only (as opposed to all sorts of trichotomy: ibid. 301, 502, 657, 900, 902, 1440–41, 2828). The approval by the ordinary magisterium of the Thomistic theses concerning the human soul (ibid. 3613–22) must be understood as a favorable reception of them as one of the best illustrations of the mystery of man.

In conclusion, the proposition could be declared as dogma.