The proper, hardcore way is to use the bread-flour mix as the leavening agent. But you can cheat and start with a bit of yeast and water the way you usually do and add the fermented mix.

The mix is sometimes called the levain. You can keep a bit every time you bake and feed it and it will live for years and years. It's a bit like having a pet ;) "Sorry, I have to go home and feed the levain" is the perfect excuse if a creep in a bar has a threesome proposition for you and your BFF/GF of your partner... Saves you having to punch him!