So how to import MIDI files? Just drag and drop a .mid file into Soundtrap and go crazy with any Soundtrap instrument! :)
Exporting MIDI files is also possible in Soundtrap! You can find the export option in the track options menu. Click the dot-dot-dot in the track header (it appears on hover) and select Export->Export MIDI to file. You can export directly to Flat.io and Noteflight as well!