FL Studio allows you to open one MIDI file at a time, using the the Import > MIDI file option. If this file has multiple tracks the pop-up dialog will allow you to select and load these into separate Channels. If you want to load several MIDI files into separate Channels then please follow the procedure below:
Select multiple MIDI files in a Windows file browser. Drag and drop them to empty space in the Channel Window to load them in empty Channels. Right-click the channel selectors and press Ctrl+X to cut the notes. Select the channels you want to send the notes to and press Ctrl+V.
The Image-Line Support Team