Rails Engines: How to generate a controller for Rails engine?

We have three Rails engines maintained in a single application. They are placed in vendor folder,

vendor/
   - Engine1
   - Engine2
   - Engine3

How can I generate controller for Engine1 from my root folder as we normally create controllers? For example,

> rails generate controller Engine1:users

Note:- I could create controller by moving around the folders. That is move to Engine1 folder from root app and generate controller, which works fine. But every time moving around from my root app and generating gets bit hard.