One approach is to use available dictionary files and add entries to them. They are just the text files, so you can edit it directly, manually or programmatically. On little problem is that the files I saw so far were in KOI8-based encodings, not in Unicode (so, different encoding for each language), but this problem is solvable. You can convert the file in Unicode, modify them and convert back, for example. I don't know if Unicode can be used.
You can find information on structure and development of dictionaries here: http://lingucomponent.openoffice.org/dictionary.html