Now I want to add padding to my mp3's...
So, I'd suggest re-ripping to WAV. Edit the WAV and save-as MP3. (Since WAV is not "easily" tagged, the ripped WAV files might not be tagged, and you might have to do it manually.)
mp3DirectCut is FREE. I'm not sure if mp3DirectCut can add silence, but it can join files. So, you can make a 1-second silent MP3 (with a regular audio editor) and join the files together.
To create a fade or a silence for a selection you can use the gain dialog in the Edit menu. A gain area can also be created by the Normalize function in the Edit menu.
How can I insert a piece of audio or some silence?Insert works olny if you compile a file by appending pieces. To add silence record a piece of it and save some seconds to a new file, then append something to this silence. And so on.