STRICTLY AMATEUR QUESTION: Creating Trailers a la iMovie?

You can make "templates" out of each one of the trailers and make edits in FCPX (add transitions, clips, add effects, etc.) You cannot change the title text once you "port" them, so you need to do the text you want beforehand (or remove the text if you have the tools to replace it.)

Create a trailer in iMovie (pick the trailer) then File > Convert Trailer to Movie. Select Send iMovie Library ### to Final Cut Pro.

Everything is sent to FCPX like a storyline with placeholder clips, title background clips, transitions (where used), and audio.

Title clips are placed as you might expect regular titles to be in FCPX, but they will not have editable text. However these can be replaced in FCPX if you don't mind taking the time to find a substitute (usually in the Build In/Out category).

There are tools available that will let you edit the titles (in a sense — add text to), for example, in the following, "Master of disguise" is from the iMovie trailer template and I added "The Greatest International" inside FCPX... and matched the typography.

Some of these look like they would be fun to play with -- thanks for bringing them up (I don't generally use iMovie at all).

Although templates allow one to work quickly and easily, it is at the expense of creativity and uniqueness.

Full creative control allows one to match the look of the trailer to the project for which it’s a trailer. I think consistency of style is very important. 

I like to study the structure of commercially made trailers. The brevity, structure and requirements of the medium impose a nice creative discipline. 

iMovie was a good starting point but now I do it all in FCPX: editing, sound, transitions, effects, text, etc. 

So, I'd encourage you to roll your own. They’ll be better and you’ll have more fun.