A bit OT from the Indian/Thai discussions - I've spent some time in the competition BBQ world, and about half the folks (or more) in the circuit take somebody else's commercial sauce or rub, then modify/customize/personalize it with additional ingredients. The recipes look like "start with 1 cup KC Masterpiece (or Kraft, or Heinz ketchup, etc.), then add ..."

I suspect lots of very good cooks do this in many cuisines.