OK, I think i finally get what you are asking. I too thought you wanted to have the FCP7 transition available in FCPX.
Unfortunately, you can not simply drag the transition in the time line to the effects tab. When the effect was originally added to the timeline, FCP, essentially, reads the commands in the effect, and saves that information in the project file. The original effect, can be removed from the system without affecting the transition on the timeline. You can even email the FCP project file to another person, and they will have the transition, intact, on the timeline.
What you may be able to do is save the effect in the Favorites folder. Right-Click (CMD-Click) on the effect, and select copy. Then double click on the Effects>Favorites folder to open the Favorites folder as a window. Then Right-Click in the Favorites window, and Paste the effect. If that works, you may now be able to use the effect in other FCP projects.
Otherwise, a quick google search for the transition name may give you the website of the author, who may still be able to get you a copy.
Once you have a physical copy of the file, place the file in the "/Library/Application Support/Final Cut Pro System Support/Plugins" folder.
Hope this helps you out.