It is quite easy to extend Zimbra's spell dictionary word list. Aspell needs a file in the user's home directory named ".aspell.fr.pws" (for French, replace "fr" with the language of your choice) containing a header and a word list, one word per line. Words with special characters must be encoded in ISO-8859-1.
I wrote a small python script to generate this file. Actually, the file is created in /tmp/dict.txt and has to be moved manually to /opt/zimbra/.aspell.fr.pws and then 'chown zimbra:zimbra .aspell.fr.pws'.
The scripts takes a file with a word list as argument and generates the proper aspell file. If the file is in UTF-8, it will automatically convert it to ISO-8859-1.
As usual, all suggestions and improvements are welcome.
The script is enclosed but can also be downloaded from Fichiers WHM