http://hackaday.com/2005/07/20/ds-serial-interface/ has a serial interface as an option.
One of Datel's media player for the DS was a hard drive reader and thus you could have CF. Equally it is for the GBA but
http://reinerziegler.de/GBA/gba.htm#great GBA hardware might be of interest.
I am not sure what practical value it has -- I am sure you could get a bunch of android devices with USB host, and also things like the raspberry pi. If you really wanted it for use with the DS you could probably make something to host a card (not sure what various NAS programs for the raspberry pi are doing but I would start there) and its own network (many NAS programs/operating systems/plugins will have the option for FTP at least) and have conventional DS wifi based homebrew join and pull or push files wherever you wanted them.