good request! the workaround for this is to record your performance freely and manually map it with the time tool ( you have to lock to time your track before and move the bars to the transients or midi note, the "tab key is useful in this case ), the rest of yours tracks, if are beat related, will follow the new time. after you can reset your "main Track" to beat and edit the time variation to adjust it or to manage the excessive exscursion of the bpm variation.