Just to add a comment to Didgeridrew's great summary, the real danger of adding anything other than flour and water to starters is contamination. A sourdough culture consists of a symbiotic community of yeasts (which make the bread rise) and lactic acid-producing bacteria (which make it sour). Like almost any natural fermentation process, sourdough depends on a selective environment with conditions that will tend to allow certain microorganisms to flourish, while not allowing other ones to grow.
Messing with this balance is asking for trouble. I have heard of people trying to add everything from sugar/honey to milk to fruit juices to various foods or spices to their sourdough cultures. While it's possible it might work long-term, the more likely scenario is either (1) the thing you add has undesirable bacteria/mold/whatever already in it and will eventually spoil your culture and/or (2) the added nutrients from these new additions destroy the balance between the "good" bacteria and yeast and allow other bad things to start growing.
If you want to influence the final bread dough and incorporate some other ingredient into your starter early, I would suggest a multi-stage process of building up your dough. Many people just mix their starter directly into the final dough, but you can also begin with a much smaller amount of starter and gradually add flour and water to build up to the final amount of starter you need for the recipe. Along the way, you can introduce other components into the starter (which now more accurately would be called a "pre-ferment"). If you do this in stages, you might be able to add these ingredients hours or even days before the final dough mix, which will allow plenty of time for those ingredients to influence the final bread. Meanwhile, you save the rest of your (uncorrupted) starter for future batches.