Hi,
Currently this is not possible and the main reason is because the app is primarily an arranger, so notes must be imported in the right chord (other than drums).
STY files contain all that information that's why the import is restricted to STY format even though STY is the same as a standard MIDI file with these additional markers.
So, to import a standard MIDI file in MIDI Arranger correctly there will have to be a conversion to a style which is a very complicated task.
You are not the first user to ask for this, there were few others who are primarily using MIDI Arranger as a simple sequencer, no chord recognition and transposition involved. My plan is to create another, simpler App, that will be a pure sequencer, no accompaniment functionality. In that one I will use the standard MIDI format for all files, but this is a long way down the line.
Thanks.