Transforming mp3 files to a Guitar Pro compatible format is not an actual conversion. You will have to pass your mp3 file through a few phases, in order to obtain a .gtp, .gp3, .gp4, .gp5, .gpx file. First of all, download and install a reliable mp3 to midi converter, such as Dream MP3 to MIDI Converter. After you transform your mp3 file to midi format, open Guitar Pro and insert the newly created sound file. Directly from the application's interface, save the file in a Guitar Pro format for its later use.