I looked in the AVR 328 datasheet under the USART section. And indeed the hardware USART can be made to handle 9 data bit sending and receiving data transactions. This is not related to enabling the parity bit, as it appears if you want to utilize the 9 bit mode you can't also have parity enabled, one or the other or neither option are your choices. At least that's my read on the subject. It is however a little complex and requires extra steps to set up the 9th bit before sending the character out, or reading the 9th bit before reading the 8 bit character on received characters. This is certainly not something you can configure with the standard arduino serial commands. You would have to drop down to direct USART register controls and write your own serial library to accomplish 9 bit transactions. Like I said, a little complex and obscure but it is covered in the 328 datasheet in the USART section.
Lefty