One way is to find a USB device that works with Linux. And I guess you already have that. That will be the easiest route, once the USB performance issues have been ironed out. Then you can almost copy your PC setup, provided you compile the necessary software.
Another way is over the SPI port. That seems to work well, but SPI connected audio hardware is not available off-the-shelf, AFAIK. There are at least two 8 channel AD/DA boards being developed and one of them seems the right kind for pro audio.
I'm not certain writing 8 channels at 48 KHz/24 bit to any SDHC card will function immediately. But it's exactly what for instance a Zoom R16 does with a lesser processor than the RPi.