In all sense you should be able to go to an existing matrix/parent item that has already been created with child items, go to the MATRIX tab and choose some new options from the same matrix list, click ADD ITEMS, and it'll add these new options to the existing list on your site.

But it doesn't work this way. It just gives you an error that doesn't make sense. The only work-around I've ever known was to delete ALL the child items, then go back to the parent item, select all the options you want (including the "new" ones you wanted to add), then hit ADD ITEMS. It then adds all of the child items back and adds the new ones. But this doesn't work if you've had orders on some of the child items because then you can't delete them!