Perhaps there is no "sd" to "cf" converter, but I have seen converters from "sd" to "sata" and "ssd drives" could be great to expand recording time for raw dng files three o four times.
http://www.cablematic.com/search/sd-sata/2_dot_5-SATA_hyphen_HDD-Adapter-_open_parenthesis_1xSD-to-SATA_close_parenthesis_/?pag=1
http://www.cablematic.com/search/sd-sata/2_dot_5-SATA_hyphen_HDD-Adapter-_open_parenthesis_1xSD-to-SATA_close_parenthesis_/?pag=1