Does anyone know if there is a free open-source solution to convert KORMARC (Korean MARC) into MARC21 (aka USMARC)?
While I'm not certain it has KORMARC support, you may want to try USEMARCON if you can find a mapping. From the USEMARCON page:
USEMARCON facilitates the conversion of catalogue records from one MARC format to another e.g. from UKMARC to UNIMARC. The software was designed as a toolbox-style application, allowing users with detailed knowledge of the source and target MARC formats to develop rules governing the behaviour of the conversion. Rules files may be supplemented by additional tables for more accurate conversion of MARC-specific character sets or coded information. The tables and rules files are simple ASCII text files and can be created using any standard text editor such as MS Windows Notepad.
Also, this thread from the Ask a Korean Studies Librarian Google Group might be useful, particularly the following message:
Library of Congress once tried to download records from the National Library of Korea (NLK) to use as order records. LC wrote a specification and developed a in-house program to convert KORMARC to USMARC. Since NLK records only provide script, LC used a transliterator to provide romanization for Voyager system developed by non-LC programmer. The feedback of this method is not very positive by LC staff. ... In stead of converting KORMARC to USMARC, a few research libraries including LC is currently using MarcEdit with Excel spreadsheets which are provided by Korean vendors based on contract. Vendors provide both Korean script and romanization for several elements of MARC fields (ISBN, title, author, publisher, place, series, etc.) in different columns of spreadsheet for your order items. It sounds a lot simpler to set up initially. And once MarcEdit is set up properly, it creates MARC records.