Your soul is what you are when you live your life. Notice that the Bible also declares animals as "living souls". And this is correct, because actually animals also have their own personality. Some dogs are friendly, others calm, others brave, and this is regardless of their race and age.
Then, the soul is not something that will enter in your body, but something that will be according to your behavior. When the conception happens, the new being has and is a soul already. He has a soul because he will act according to his thoughts or instincts. He is a soul because its him. Like saying you have your own DNA, and you are what your DNA is.