How do I re-add non-MAS apps to the launchpad?

I have no problem doing that with the apps downloaded/bought from the Mac App Store. I can't do that with apps that are from an external source (ie: a website, an online store that sells DLC, an optical disc, etc.).

I tried to drag a non-MAS app to the Launchpad icon on the dock. By the time my cursor hovered over the Lauchpad icon, the icon went dark and it gave me the impression that the non-MAS app would be added to the Launchpad. I released my mouse button, checked out the Launchpad and noticed that the non-MAS app's icon doesn't show up on the Launchpad at all. Like I said before, I have no problem doing that with MAS apps.

Is there any other way I can add a non-MAS app to the Launchpad? Do I need to download any third-party software? (I already have Launchpad-Control installed and it won't let me re-add any non-MAS apps.) Do I need to resest my Launchpad configuation?

Please help me with this. Thank you for your time.