Yes, that seems to be a very confused way of putting things together.  The only way to do this is to reload each .flp and bounce them down to .wav's... then load up a master .flp and drag each of the wav's into the playlist (either upper or lower portion, depending on what version of FL you have)... then arrange from there.  I'm not sure why you would choose the method you have for producing but each to his own... I would highly recommend that you complete your tune in one .flp in the future to avoid confusion.  If resources are an issue , consider bouncing each part to .wav as you complete it and reimporting...