I suggest you find all those images on your hard drive and do the organizing before you import into LR. AFAIK LR doesn't search your drives for images like iPhoto does. In fact I just checked and there is no option to Search your computer for images. LR doesn't try to take over your computer and images like the Apple Mac software does.
It is up to you to point LR to the folders your images are in during the import process.
So open the File manager program and create some folders to store your images in. I do this by year, month and day the images were taken on but you can do it anyway you like. Copy and or move your images into the newly created folder or folders and then use the LR import dialog windows to import them into LR. you would then select Add in that dialog. You could also use LR to create the folders and then use the Move option to move specific images into different folders. But I find it easier to do this with a File Manager before importing to LR.
I've got stuff scattered all over from poor habits. I was accustomed to Image Expert software that came with Toshiba camera years ago grabbing photos and putting them in folders. Over the years I allowed camera, smart phone upgrades, etc to make shambles of somewhat organized photo library. Looking for easy fix that doesn't exist. I'm gonna have to bite the bullet and do as you suggest. Thanks!