Isso fará o que você quiser. Vincular à chave de sua escolha:
(defun switch-to-minibuffer-window ()
"switch to minibuffer window (if active)"
(interactive)
(when (active-minibuffer-window)
(select-frame-set-input-focus (window-frame (active-minibuffer-window)))
(select-window (active-minibuffer-window))))
(global-set-key (kbd "<f7>") 'switch-to-minibuffer-window)