Sourdough starter from regular yeast?
Yes you can create sourdough from regular yeast, but it's not necessary. Ordinary flour comes with a trace of yeast. In the San Francisco area, that yeast is famed. Just mix flour and water 50/50 to make a batter-like mixture and leave it in a jar on the counter. Put a lightly fitting lid on the jar.

Add more flour and water every 12h (give or take). You should be doubling the initial amount each time, so 1 teaspoon, 2 teaspoons, 4... 

After a couple of days you'll see bubbles developing in the sourdough. Once you're at that stage, you can reduce the amount of care. 'Refresh' the sourdough once a week: mix, take out 1/2 of the sourdough, add the same amount back (50/50 flour/water as before), mix. According to the Wikipedia, this can be kept at room temperature.

Sourdough is a mixture of flour, water, yeast and bacteria. When unattended for some time, 'water' forms at the surface. This liquid contains some alcohol that adds to the flavor of the dough. It's part of the sourdough and should be mixed in when refreshing the dough.