In the meantime, I found out that you can also use the official Pocketbook dictionary converter to convert dictionaries from xdxf format. The public domaon Webster from 1913 is also available in this format. Here are the instructions:
1. Get a dictionary from http://sourceforge.net/projects/xdxf/files/dicts-XDXF/
and unpack it. You will need a tool capable of unpacking .bz2 archives. The Webster is in the file comn_sdict_axm05_webster_1913.tar.bz2.
2. Get the PocketBook converter: http://pocketbook.com.ua/file/DictionaryConverter.zip
3. Unpack it with all subdirs to the same directory as the dictionary
converter.exe dict.xdxf eng
That's all! The resulting Webster is about 6.3 MBytes.