Hi ChecMark,
vapors is correct. The software which would best suit your needs is WavePad. MixPad is focussed on taking multiple audio sources and then combining them into one final mixed file. WavePad is more focussed on editing a file and saving the changes.
What you want to do *can* be done in MixPad, but you would need to do it to one song at a time. But as has been said, it's not really designed that way. WavePad wil allow you to apply a fade in/out to your tracks as a batch process which means that once you have set the fade in/out time in the way you want, it can be applied to any file quickly.
Regards,
Ben