I created an additional mode "move" in ~/.i3/config
 ##
# move floating windows with keys
bindsym $mod+m mode "move" focus floating
mode "move" {
    bindsym $mod+Tab focus right
    bindsym Left  move left
    bindsym Down  move down
    bindsym Up    move up
    bindsym Right move right
    bindsym h     move left
    bindsym j     move down
    bindsym k     move up
    bindsym l     move right
    # back to normal: Enter or Escape
    bindsym Return mode "default"
    bindsym Escape mode "default"
}
 To start the "move" mode press Alt+m, then the first floating window gets focus. 
To move focus press Alt+Tab. You can either use the arrow keys or hjkl to move the floating window which has focus. 
All keybindings are according to my preferences but can of course be changed to your liking.