It appears tha mp3 to bnk conversion is sought for the purpose of encoding audio files to BINK video format, perhaps as an attempt to import audio track directly to existing video file.
It might be possible to use just the audio track (.mp3) and create blank BINK video (.bnk), i.e. one that has only this audio track and no video. However, the purpose of this conversion would be pretty useless as BINK is generally used for game videos that typically have no sound as all which is kept in separate files.
Updated: April 20, 2017