Adding a keydown to this function?

Since KeyDown is a event of mouse you could access player from hit by using the :GetPlayerFromCharacter.

function onTouched(hit)
    local Humanoid = hit.Parent:FindFirstChild("Humanoid")
    if not Humanoid then
        return;
    end

    if not script.Parent:WaitForChild("ButtonEvent") then
        script.Parent:Destroy()
    return;
    end
    local pl = game.Players:GetPlayerFromCharacter(hit.Parent)
    local mouse = pl:GetMouse()
    mouse.KeyDown:connect(function(key)
        if key == "e" then
            script.Parent.ButtonEvent:FireServer()
        end
    end)
end

script.Parent.Touched:connect(debounce(onTouched))