You can't. You would need for example, adobe premiere pro or apple's Final Cut Pro.
How do I blur the background of a video in iMovie without blurring my face?
The point, OP, is that this is something that's not for any video editing software, really, but for video compositing software.
How do I blur the background of a video in iMovie without blurring my face?
How do I blur the background of a video in iMovie without blurring my face?