There is an auto import folder inside the iTunes Media folder.
You can make an Automator Folder Action that moves the mp3s into that auto import folder. When you next open iTunes, the files will be imported.
Open Automator and choose Folder Action.
Make it look like this:
Save it and when music files arrive in the Downloads folder, they will be moved to the Automatically Add to iTunes folder.
That path is ~/Music/iTunes/iTunes Media/Automatically Add to iTunes
The two actions I added are in the Files & Folders Library.