Move existing app service into app service environment
I assume your existing apps have already been deployed into existing App Service Plan? If so, you cannot move, all you can do is to clone it into a new ASP which has been deployed into your ASE
Given the fact that an ASP’s location (within an ASE or in a shared public location) cannot be changed once it is created, you have to clone/duplicate the deployments when you scale your apps in this approach. It also implies that the order you create resources is ASE (optional), then ASP, and finally add App Service apps into an ASP.
Or you can see my ASP/ASE learning at my blog