You can now import environment variables across projects from within the UI.

1. Navigate to your project settings page of the project you'd like to import environment variable into and navigate to the "Environment Variables" tab:

https://app.circleci.com/settings/project/github/{ORGANIZATION_NAME}/{PROJECT_NAME}/environment-variables

2. Click the "Import Variables" button:

3. Select the project you would like to import environment variable from:

Please note, you must be following both the project you would like to import environment variables to and from.