Christoph Schäfer asked on kimageshop:
You were right, renaming RW2 to RAW enabled the import. As it seems you are using libraw, so Krita should list all supported formats -- probably a tiny update to the file handling code, and the same would be necessary for the palette formats.
plugins/impex/raw/3rdparty/libkdcraw/src/rawfiles.h holds libkdcraw's list of supported RAW file extensions, so I copy-pasted it into the relevant files.