I don't know if there are specified fonts just for iMovie and I have never tried this, but i believe if you Control-click on iMovie, you get a contextual menu.
Select "Show Package Contents". This opens a window with a folder named "Contents". Open this folder and then open the "Resources" folder and within that, the "Fonts" folder.
All of the iMovie fonts are there.
Copy the font you want out of that folder. Then open Font Book. Under the File menu, select "Add Fonts..." and navigate to the copied font. The font should be available in all your applications.
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iMovie Fonts
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