You can try to use the Monitor Folder option to do this for you. Go to the Options menu at the top left of the Zune software and select Library --> More Options. Click on the Monitor Folders... button. Select Add Files Previously Deleted From Library and deselect Don't Add Songs From A Duplicate Media Library. Ensure the new folder is one of the monitored folders and then click Ok.
It should now scan the new folder and pick up all of the MP3s. Once finished, you can toggle those options back to whatever it is that you want.
If they still do not show up, you may want to make sure the files actually have an .mp3 extension. When doing some batch renaming in the past, I had inadvertantly removed the "." in the file name so they didn't have a valid extension. You can see this easily by viewing them in Windows File Explorer. If the file doesn't have an icon that shows it is associated with your default mp3 player -- my default is Zune so the Zune icon appears for all -- then the file is not being viewed as an .mp3.
Hope this helps.

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