You can do this in Final Cut Pro X, but not iMovie.
However, if you have Apple's Keynote App, you can put text anywhere you want. And Keynote has a lot of text effects. You can create a text effect in Keynote against a green background, then use the green screen effect in iMovie. A gradient background between two similar shades of green works best. Then Export the Keynote project as a QuickTime Movie with Timed Advance. Import this movie into your Event. Then you can drag the title onto your project and drop it on the frame where you want it to start. In the Popup Menu, choose Green Screen.
As a hint, make a still image of the iMovie frame where you want the Title. This will help you position the title properly. Also, make your Keynote the same dimensions as your movie e.g. 1920x1080.
Here is a sample from Keynote using a typing effect.