A transition would be fading from one video into another. If you want the same video to continue afterwards, you can just place the generator "on top" of your video.
If you really want to do a transition, you will have to start off with a transition project for Motion, not a simple Motion project in the "New File" dialog. Converting afterwards is rather complicated.
In your case, if what you are looking for really is a transition, I would recommend you to create a new Motion project of type "Transition" and then Copy-Paste everything you need from your old project.
This is for example a quick tutorial on how to create transitions:
How to create your own transition for FCPX