I have FL Studio 8. So can i mix stuff I record with stuff on the piano roll?
yeah. But it's better export all of your tracks and edit and record in a proper mixing/recording program (get reaper for this)
Agreed, recording forum, but just for a quick bit of help, it sounds like you're talking about mixing MIDI with WAV, and that's quite possible. Just import your MIDI track into a mixing program and your WAV track then mix them together and export the finished product, just like you'd do it with all WAV tracks. Also, unless you're using some kind of MIDI that isn't the default Windows MIDI sounds I'd advise not using MIDI as it generally doesn't sound that good.
if hes using FL he would likely be using synth plugins and not the normal MIDI soundfont