Your procedure won't work when you use a starter as you are. Instead, after you divided the dough into pieces, place each into an oiled, plastic container and cover lightly with a piece of foil, and set aside to ferment at room temperature. I can't say how long this will take as I don't know anything about the type or strength of your starter, but it could take anything from 5 to 6-hours at the short end to 1 day or more at the long end. Watch the dough balls, when they have grown somewhat, and are resilient to the touch, try opening one up, if it opens easily, they are ready to go, if not, you will need to wait longer.
Can you add yeast? Yes you can, and then you don't need to wait nearly as long, but the yeast will become the dominant microflora, completely overwhelming the bacteria in the sour, hence your sour flavor will be lost.
One last thing, most sourdough starters are used at around 20% of the total flour weight. This is assuming a wet, active starter.
Tom Lehmann/The Dough Doctor