Yes, just like the JockyW wrote. You need to enable ide support in the kernel and compile as modules.
You can also compare .config files on the kernel dir - they contains all options enabled/disabled in the kernel. Try to figure out minimalistic options to reduce size of the kernel module (we do not need tape or floppy support, etc) - ASUS guys do not care about this.
So, modify kernel config by adding new modules, compile firmware and check that they're insmoded ok and hdd could be manually mounted. Once done we can modify remaining parts of the firmware to do automatic insmoding.