Customizing/extending language layout

[vzaliva]

I type mostly in English and Ukrainian, but occasionally I have to type in russian. Now the solution is to install three languages, which makes switching cumbersome (3-way instead of flipping). On Mac, Linux and Windows, there is a popular Ukrainian keyboard layout which adds four missing russian characters via the AltGr key (or pressing and holding a button on virtual keyboards):

https://github.com/OleksiyRudenko/Ukrainian-Enhanced-Keyboard-WIndows

Is it possible to implement something like this for Fleksy?

[vzaliva]

FYI: GBoard have this https://www.reddit.com/r/Ukrainian/comments/180aoim/why_my_iphones_ukrainian_keyboard_consists_of/ but I would rather use Fleksy :)