This is not compatible with Catholic teaching.
Mormons, as well as some other religions (esp. those associated with New Age beliefs) profess that the human soul pre-exists our human body. According to this Mormon website, "Before you were born on the earth, you lived in the presence of your Heavenly Father as one of His spirit children."
However, Catholicism proclaims that at the moment of conception, a new immortal soul is created and...this is beautiful--the universe is changed forever! When a new human person is created, a new soul comes into creation.
From the Catechism: "The Church teaches that every spiritual soul is created immediately by God"
"The traditional philosophy of the Church holds that the rational soul is created at the moment when it is infused into the new organism."http://www.newadvent.org/cathen/04475a.htm
And from the fifth ecumenical council of Constantinople in 553: "If anyone asserts the fabulous pre-existence of souls, and shall assert the monstrous restoration which follows from it: let him be anathema."
(Here, "fabulous" is an archaic way to say "mythical" or "relating to a fable". And "anathema" is another word for "excommunicated". It is a harsh term, to be sure, and perhaps if it were to be stated today the language would be gentler, but the take home message is clear: we should not believe in the pre-existence of the soul.)
More on "anathema" and "excommunication" here:
"Does the Church condemn those who disagree with its teachings?"
http://www.catholic.com/quickquestions/does-the-church-condemn-those-who-disagree-with-its-teachings
