* 33 million Americans suffer from overactive bladder * Analysts forecast more than $200 mln in additional sales * Allergan sees 2012 Botox sales of $1.76 bln to $1.8 bln Jan 18 (Reuters) - The popular Botox wrinkle treatment made by Allergan Inc has been approved to treat adults with overactive bladder who cannot tolerate or failed to be helped by other drugs for the condition, the U.S. Food and Drug Administration said on Friday.