Civilization VI – Asynchronous Multiplayer (aka PitBoss)

Unfortunately, there doesn't seem to be an easy way to do this in-game. However, much of the code isn't hidden, but is in easily modifiable .lua files, specifically WorldInput.lua.

https://www.reddit.com/r/civ/comments/58q3qj/psa_guide_on_how_to_enable_wasd_andor_other/

This link contains several examples of how to add new hotkeys. For example the code snippet

if (uiKey == Keys.YOUR_KEY_HERE) then
    local unit:table = UI.GetHeadSelectedUnit();
    if unit ~= nil then
        UI.LookAtPlot( unit:GetX(), unit:GetY() );
    end
end  

can be used to make the 'C' key center the view on the current unit, and

if (uiKey == Keys.W) then
    UI.SelectNextReadyUnit();
end  

to cycle units.

In the end, the range of new hotkeys is determined by the functions accessible, and I don't see a list of functions yet. Luckily for you, there's already examples of what you want to add.

As of now (after Fall 2016 update), there's nothing official you can do. There are related Casus Belli, such as a Holy War if they've converted one of your cities, but I believe this requires you to have founded a religion of your own.

My typical response is to beat them up and live with the warmongering penalty, but perhaps that is not always the answer.