This is probably a request you should make upstream: https://github.com/ibus/ibus/issues/ Available XKB layouts seem to be hard coded in this file: https://github.com/ibus/ibus/blob/master/engine/simple.xml.inI don't know the criteria for the selection of XKB layouts; actually fr(latin9) was deleted as "useless" a few years ago...
https://github.com/phuang/ibus/commit/661bb47f -- Gunnar Hjalmarsson https://launchpad.net/~gunnarhj