Preferred Stocks are really bonds, without a maturity date. They are "stocks" for accounting purposes only. The don't appreciate--or depreciate--based on earnings, like common stocks do. Consequently, it's pointless to split them.
*Convertible Preferred Stocks are a slightly different animal. The offer the investor a lower interest rate than regular Preferred Stocks, in exchange for the option to convert the Shares into common stocks. Consequently, if the underlying common shares do split, there is a mechanism in the terms of the convertible to adjust the convertibles conversion value accordingly.