Thanks for your feedback, for the moment no way to "re-inject" the custom admin.
You'll have to run both (admin and back-end) separately.
(personally, I even deleted the admin plugin from my back-end once I had ejected the plugin)
In any case, you should know that this is a temporary solution for customizing and that in the long term the Medusa team intends to offer a way to customize the admin-ui "natively" so as not to miss any potential updates that may come in the future.