the way to do it is find the bpm the song is at (lets say 130) and then the bpm you want to change it to (lets say 100). take the difference of the two bpms (130-100=30) and then find the percentage you need to change the original bpm by (30/130=.23076...roughly 23%) then timestrech it by that percent. To find the bpm of a song automatically in Edison go to the page looking button second from the left above the waveform and click edit properties. in the tempo menu click autodetect and then whichever option fits the type of song your trying to find the tempo to the best.
No, there is no way to automatically stretch a song to fit a set bpm in fl studio (that I know of). If you want to do that, use ableton live (which actually might be a lot better for your purposes; its an awesome dj tool). Ableton automatically restretches songs to fit the mixes set tempo, plus you can warp the songs to make them fit even better.