Yes. You can create your own SD/MMC adapter for the BitsyXb.
However, perhaps a more efficient solution could be to connect your BitsyXb to the Bitsy G5 personality board, rather than reworking the BitsyXb Personality board to support this functionality.
The BitsyG5 Personality board description and schematic are available for viewing at Eurotech Forum Topic 2552.
Drop us a line at support.us@eurotech.com and we can work through your individual situation in detail.