How to uninstall not App Store apps using Launchpad?
It is possible but it's a bit "hacky".


Open your Applications Directory in Finder
Find an app the was installed from the Appstore.
Right click on the .app and select Show Package Contents.
Copy this directory (_MASReceipt).
 
Now repeat the above steps or write a script to place that directory in 
all of the non Appstore apps by selecting Show Package Contents and pasting the copied directory.