Yes. You can develop the admin panel as a new, separate service/module of your app.
Services have code-level isolation (see Service isolation) so the choice of the GAE environment (standard/flexible) or the programming language used is done at the service level and can differ from a service to another inside the same application. See also is it possible to have a hybrid PHP+Java web application on Google App Engine?
As for the DNS mapping you can keep both services under the same domain, but you will need to establish clear, non-overlapping URL path patterns in your app.yaml
(or equivalent) config files to be able to route the incoming requests to the proper service.
For example:
You may need to use a dispatch.yaml
file for inter-module routing, see Routing with a dispatch file
You may be running in a few problems as first. When debugging pay attention to your catch-all app.yaml
patterns like url: .*
and check your logs (for all services) to see if the requests are being handled by the right module.