I have N MP3 files (N files for example). I need to superimpose, or mix them into one. Is there a special LAME function for performing such an operation?
LAME is the wrong tool for mixing -- mixing isn't encoding.
Do the mixing with a tool such as sox, then encode the mixed stream with LAME.
Use Audacity to mix them, and Audacity will then use the LAME encoder to encode the mixed audio as MP3.