Opps. Looks like multi selection is only available when you create the movie. Once the movie is created, you can only add one photo at a time.
Start a new movie, choose Photos in the left column and select all the pictures you want, then tap Create Movie at the bottom of the screen. It creates a Ken Burns type slide show, which I doubt is what you want. There doesn’t seem to be a way to tweak this.
You may need to use a different app to do what you want. A search for ‘Time Lapse’ in the App Store shows several possibilities.