You don't need to alter the original transition and it's not recommended you try without a Save As with a different name first.
All you need is a "slug". A slug is just blank video. You can create a slug by creating a Generator and immediately saving it as "Slug", then apply it to the storyline and apply the transition between your clip and the slug generator. If you're using a connected clip, turn it into a Secondary Storyline (Command-G) and place the slug after it, then apply the transition.
You can also download a pre-made slug here: http://sight-creations.com/free_stuff/slug4fcpx.zip
If you're using Safari, you'll find the unzipped files in your Downloads folder [Slug folder and Slug4FCPX readme].