The simple answer is, there is a simple and direct path. The Pope as the Supreme Interpreter & Guarantor of Faith can proclaim it a Dogma ex.Cathedra any given day and it will be binding to the Universal Church.
Pope Francis INFALLIBILITY is a Dogma decreed in Vatican I in 1870.
The voice of Peter is the Voice of Christ.
"He who hears you hears me" (Luke 10:16),
Peter was given the Keys to God's Kingdom and whatever he bind on earth is bound in Heaven.
and "Whatever you bind on earth shall be bound in heaven" (Matt. 18:18).
The word that you extract from Humani Generis also comes from a Pope and that word is already applied to Mary's Immaculate Conception which is also a Dogma.
ENCYCLICAL HUMANI GENERIS OF THE HOLY FATHER PIUS XII
There's is no impediment for Pope Francis to proclaim it a Dogma using the words "the soul was immediately created at the moment of conception".CCC365 & CCC366 had a good explanation to support this.
The only problem is, which is a very big problem is, it will exclude Jesus the Author of Life.
Council of Nicea decreed Jesus was not created but "begotten" co-substantial with the Father and the Holy Spirit.Light from Light and God from God.
For it to become acceptable as Dogma the wordings must include the humanity of Jesus that was offered to all mankind for the Salvation of souls.
My suggestion would be like this to include the human but divine soul of Jesus Christ;
"All souls was immediately INFUSED at the moment of conception or at the moment it was formed".
I used the word "formed" also, so as not to forget our First Parents who are "formed" by God.
Genesis 2:7 New King James Version (NKJV)
"And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living being.
If the Theologians can accept the word "formed" as similar to "conception"
as Jeremiah also used the word "formed" by God.
"Before I formed you in the womb..." (Jeremiah1:5)
Therefore, Adam & Jeremiah was both "formed" by the Creative Hand of God.
It is also noteworthy to ponder the words of Archangel Gabriel to Mary;
"Behold, you will conceive and give birth to a son, and you shall give Him the name Jesus. "(Luke1:31)
Archangel Gabriel belong to the Choir of Chrubim like Lucifer who knows the Secrets of God, but Lucifer when cast down loses the Light of Truth.
So, there is a Great Wisdom why Archangel Gabriel uttered the words "conceive" as he can simply omit that part and said, "you will bear or give birth to a son". But mentioning the word "conceive"it becomes a point of reference and understanding where life would begin and it is a Divine Truth as it was not spoken by man but by an Angel that hold the position as God Messenger.
Although Pope Francis Infallibility and Canonical Power & Authority plus charisma that the Holy Spirit is guiding his Divine Office can proclaim it, I wholeheartedly believe that He will avoid declaring it a Dogma much more the Holy Spirit will inspire him to bind all the faithful into that decree in these Time of Mercy.
Proclaiming this as a Dogma would send a lot of people to Hell as Abortion was being practice by millions of people and since a Church decree is bound in Heaven, the Just Punishment of God to all guilty souls will definitely be terrible as God explicitly said what punishment awaits those who kill and murder a life that God gave to every human being.
Fortunately, the Holy Spirit inspiration to Vatican II New Evangelization Mission in the Third Millennium is centered on the Divine Mercy. So this powerful Dogma although will convict and tied the hands of all catholic involve in the practice or force to make a choice on abortion to face a clear ex-communication even automatic ex-communication.
St.John XXIII opening the Vatican II, St.Pope Paul VI issuing Humane Vitae and St.John Paul Evangelical Vitae and as the instrumental person to revive the Divine Mercy revelation to St.Faustina and Pope Emeritus BXVI following the path of Divine Mercy and most especially now that Pope Francis is magnifying the Divine Mercy transforming the Church into a field hospital will not proclaim it a Dogma to avoid ex-communicating millions of souls worldwide and invite the grave punishment that God decreed on all who violates the 5th Commandment.
In closing, Pope Francis gave us his powerful reflections in these end times with this phrase;
"The Divine Mercy is INFINITE but the Time of Mercy is NOT."