Dear Biggirl, I'm not quite sure what you mean by this question, but let me try to answer a bit before vlad (who will probably have a good insight to share.)
The Church teaches that a dualistic understanding of the soul and the body as strictly separate things, is mistaken. You are one person, an embodied soul; equally true, an ensouled body. One person can't, for instance, have two souls, one now and another one later. Neither can souls migrate from one person to another, as in the false doctrine of reincarnation.
The same soul you had when you were conceived in your mother's womb, is "you" born or unborn, baptized or unbaptized, in sin or in grace, in heaven or in hell! Your soul will separate from your body at death (a very unnatural thing), but will be reunited to your resurrected body on the Last Day. It's you, forever! So no, God does not create a new soul for a person being baptized--- if that's what you meant.
Msgr Pope explains about souls here, when explaining about the resurrected body,and as usual, does a great job.
http://blog.adw.org/2010/11/what-will-our-resurrected-bodies-be-like/(LINK)
Thank-you and God Bless! I learned something new today! :)
Thank-you for the link and bookmarked it. :)