Since version 0.9.3, refdb supports risx as an additional native input format. You can employ the standard techniques of SGML and XML transformations, i.e. by running DSSSL or XSLT scripts with a suitable engine, to turn bibliographic data encoded as SGML or XML documents into risx.
Remember that refdb accepts only four character encodings for XML input data: UTF-8, UTF-16, ISO-8859-1, and US-ASCII. If your input data use a different character encoding, please use the command-line utility iconv (usually part of the libiconv package) to convert your data to one of the existing character encodings. Do not forget to specify the character encoding in the processing instructions of the input file, otherwise refdb will assume the data are encoded as UTF-8.