Xiaonan, this should be possible if the additional audio chip can be connected to the SoC without any pinmux conflicts. Unfortunately our hardware engineer is on vacation, so I'm not able to comment any further on this.
Please note that we only support the linux that comes with the TI AM335x SDK on this forum. For Ubuntu running on the AM335x, please turn to the BeagleBone community forums: http://beagleboard.org/Community/Forums
Best regards,
Miroslav