Spell check the document. When you get to a word not included click Options. Click New and create a dictionary called MyFrench.
Now when you spellcheck and come to an unlisted French word, click Add …, and select MyFrench.
You can edit it at C:\Users\xxxxxx\AppData\Roaming\LibreOffice\4\user\wordbook\MyFrench.dic - use Notepad. Editing the file like this is not supported, but works, so if you do edit it (I do to remove words added by mistake) be sure to keep to the file conventions - eg in aphabetical order.
Send MyFrench.dic to your colleagues.
Enhancing the downloaded French dictionary is best done by sending a list of suggested words to the author. If you edit the file (you can do so but you need to understand the coding for roots and optional endings) your edits will get lost when the dictionary is next updated. Be aware that the headers in a normal dictionary include a count of the lines in the file - if you add or remove words you must change the header as well.