How can I contribute to the LibreOffice dictionary?

Libreoffice is not developed by the Ubuntu development community. This is the place to go if you wish to get involved in the development of Libreoffice:

Get involved in Libreoffice

Also Libreoffice uses Hunspell as the spell-checker. You need to research how Hunspell creates dictionaries.

Regards.

The user dictionary is just for personal use. LibreOffice does not actually collect words for dictionaries, instead it just assembles existing ones. I think the best way for personal contributions is through Wiktionary from which maybe in the future additional dictionaries can be built.