Can I add an entire album to the iMovie at once?

You can add an entire album to iMovie all at once, but you need to open the album and bring in the photos as a batch. So if you click on the Photos library item in the iMovie sidebar and select to display "All My Albums", you can then open an album and select the photos as a batch in the media browser, and then click on the + sign in the first photo. That will move all the photos into the timeline at once.

-- Rich