Hello, Ebenist.
You won't be very happy reading my reply...
As far as I know, you can't edit an aready built dictionary and paste new definitions. The only solution I see to this is to convert the binary file to HTML, edit that HTML and then rebuild the dictionary again; but it looks a little complicated and not really worth the trouble.
I have been looking for a good French dictionary since I've got my Kindle. So far no luck, there is indeed a free one available but it's almost a century old (and very basic, I like a Kindle default dictionary to display a definition if you put the cursor on an inflection of a word too).
Kindle is not a perfect solution for non-english users yet. Ah, and there are no folders
I know there are French dictionaries available for PC, and maybe I could get their definitions out, but I think that would be illegal...
If somebody could provide me a source with good French definitions (besides rich definitions, inflections would be nice) I'd be more than happy to build a good French dictionary (formatting, features, etc.), so anybody can use it for free. But Google could not help me find such a source yet. French users could really use the idea from a nice open project like dexonline.ro (where romanian voluntiers added in time definitions from different dictionaries, and now it is such a mature project...).
If you have any more questions, don't hesitate.
Regards,