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That's the programmatical way to do it (although you should be careful about reserved variable names, as id is one of those)... so I'll cover the IDE way to do it.
If you have a sprite that already has sub images in it, and wish to load more sub images into it from another file, open the sprite's properties and click on the Edit Image button. Then, select the menu bar option Image -> Import Image(s). This will let you import another image into an already existing sprite as a new sub image.