I don't suspect it's currently possible but I would be pleasantly surprised to find that it is.
I'm going to use an example from Django because I've seen it used there and a similar implementation would be handy to have in Craft. Annnnd it would be helpful for what we're trying to achieve with my current project.
Example: In Django you have your project-wide urls.py (think of this as routes.php in Craft). Additionally you can have app-specific URLs (an App might be a Blog or a Portfolio). You can define the app-specific paths in the app-specific urls.py file and then you can include those URLs in the project-wide URL file. That might look like this (example from the Django docs):
urlpatterns = [ # ... snip ... url(r'^community/', include('django_website.aggregator.urls')), url(r'^contact/', include('django_website.contact.urls')), # ... snip ... ]
So a use-case:
We're developing a REST API and right now all of our routes are defined in routes.php. That's an ok solution, but we have a lot of plugin-specific routes. It would be nice to be able to define a base such as /api/ and pass the rest of the request to a plugin-specific routes file.
An idea of what this might look like:
return array( 'api' => array('pluginRoutes' => 'apiPluginName'), );
And then in craft/plugins/apiPluginName/routes.php would be a routes file similar to the project-wide routes.php file in craft/config