Doing a conversion like that would be difficult since MP3s are much more complex them midi's. I don't know if it is even possible. Here is an article I have found about doing it.
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mp3.about.com] If you convert the MP3 to wav first and then convert the wav to Midi it may work, but that would be alot of work. What might be easier is doing a search for the midi format of the MP3s you have and using them instead.