Hey there ScreamCream,
As I understand it you have created a Trailer project in iMovie, and want to add Titles to it. You are correct, in its current itteration you cannot add a title. You can however convert the Trailer into a movie, THEN you can add a Title. However, you should be certain you are finished with the finishing touches on the Trailer before hand, since you cannot convert it back into a Trailer.
To convert it into a movie use these steps from the iMovie help:
Convert a trailer to a movie - iMovie Help
Select a trailer in the browser, or double-click a trailer to open it.
Choose File > Convert Trailer to Movie.
Unfilled placeholder clips appear as grayscale images in the timeline after the conversion.
Thank you for using Apple Support Communities.
Cheers,
Sterling