Could law enforcement require access to play and appstores? Has this happened? Law enforcement could replace an app, with their own, even for one specific user

Could law enforcement require access to play and appstores? Has this happened? Law enforcement could replace an app, with their own, even for one specific user, if they have access to the system, granted by Google or Apple. I presume the signing can can be compromised, this way, but am unfamiliar to know this for sure.