When you import a MIDI file by going through the steps "File > Import > MIDI file..." a little dialog will pop up with some settings you can adjust before importing the file. Deselect the option "Start new project" which is selected by default. By doing this the MIDI channels will be added to the current project instead of opening in a new one.
You can change the sound of a particular MIDI channel by right-clicking on it and, selecting "Replace >" and choosing a different synthesizer from the list such as Sytrus or 3xOsc which are included with FL Studio. You can do the same with any third party VST plug-in you may have installed.
You can change the sound of a particular MIDI channel by right-clicking on it and, selecting "Replace >" and choosing a different synthesizer from the list such as Sytrus or 3xOsc which are included with FL Studio. You can do the same with any third party VST plug-in you may have installed.