Hello,
I noticed there is a page on Wikidata for property proposals but is there any page for property values proposal ?

For instance, I need to declare that Julien Collin was a member of the Union Régionaliste Bretonne as "secrétaire-adjoint" but this function is not listed in the drop menu of the qualifiers.

The "secrétaire" is a position held in a french voluntary association (association loi de 1901). The "secrétaire" is responsible for administrative and legal tasks ("les tâches administratives et juridiques" in French, since I am not sure of my english translation). He is not the "chief" of the association, position held by the "président" (we have many "présidents" in France, even for volontary associations). So this position has nothing to do with a secretary as the secretary-general of the United Nations. The "secrétaire-adjoint" is the assistant of the "secrétaire" (the "secrétaire-adjoint" has nothing to do with a deputy secretary).

Thanks,