Adding new yeast to old starter?

When it comes to sourdough starters, time = flavor. Over a period of months, the flavor of a starter develops depending on the conditions it is kept in.

By adding yeast, you're interfering in the natural lifecycle of the microorganisms growing in the starter. Naturally-occuring yeast are, right now, establishing a foothold and making conditions perfect for bacteria that thrive alongside them. By introducing alien turbo-yeast, you're speeding up the processes the natural yeast control...but not the bacteria. It's the bacteria that create the complexity of flavor in a sourdough starter. You might also be changing the type of yeast present in your culture. Natural yeast strains are many and varied, and industrial yeast strains are monocultures raised for one thing: their appetites.

You're not hurting the starter, it will still develop flavor and a unique profile over time, but it won't be sour. You're essentially creating a poolish, and there's nothing wrong with that, but it isn't a sourdough starter. :)

In adding fresh yeast to a starter, you are depriving the natural yeasts and bacteria of a portion of the food they would otherwise have available to eat. Think about how your pet dog would feel if he/she had to share the food bowl with a bunch of vagrant farm dogs all the sudden. It wouldn't be pretty.

If you don't have the patience for sourdough, you still can make great breads using commercial yeast and a pre ferment.

Eric

Rather than adding yeast to your underachieving starter, there are time and experience tested methods that will work. The first is to maintain a regimen of feeding the starter at room temperature on an accelerated schedule. In other words, feeding every 8 to 12 hours until the starter is healthy enough to peak on a dependable basis. Obviously, you'll have to discard or otherwise use the starter you remove at each feeding.

Another suggestion, one that has worked with my starter, is to include rye flour in the feeding or elaborating of the starter. Rye flour, especially fresh rye flour, is packed with the nutrients that will jump start your starter. 15 to 20% rye flour has always worked for me. YMMV.

Do a search in the Forum and you'll find quite a bit more information on these two ideas. The answers will be more learned and thorough than I can provide.

It would help to know the age of the sourdough starter. 

If your starter is new or lazy you may consider adding yeast to the final dough.

I second what jcking said.  But if the starter is weak/lazy because you haven't been feeding regularly for a while, be prepared that the resultant loaf can be sour.   If you don't want it, it's better to regain its vitality by feeding it regularly for several days to make it active and healthy again,  as suggested above. (unfortunately, sourdough needs patience)

I have several loaves I make with sour cultures and commercial yeast. A typical pattern is to make 1-2 cups of thick batter, or 1 cup of a dough, with a sour culture to raise it. Let this rise overnight. Then make up a dough with that, flour, salt, and instant yeast. The result will rise like a yeasted bread, but retain some sour bite. There are many recipes which do this, I didn't make the technique up!

Agree with everyone else who said not to add yeast to the starter itself, though. Commercial yeast and the yeast in the starter are different yeasts -- one will win the battle, one will lose, and you'll either end up with a battered sour starter, or just a sponge of commercial yeast.