There is no per se subtitle feature in iMovie 10. Nor can you import an .srt file directly, as far as I know. You can adapt the Lower Third or Lower titles for subtitle purposes and create a subtitle with the Text Edit app on your Mac that you can then paste into the title. Text Edit lets you adjust the spacing. whereas typing directly into the title template does not.
I have little occasion for using subtitles, but when I need one I have used the Preview app on the Mac to create a transparent screen, and then use the Annotate/Text feature of Preview to type a subtitle on the screen that I then import into iMovie and overlay as a Picture in Picture. The size and position can be adjusted.
I am not familiar with any third party subtitle apps that work with iMovie, but you could check the app store. Also, you could take a look at the Adobe Premiere Pro app that I believe has a subtitle feature. Not sure about that.
-- Rich