Official comment
When using the roll function of the DDJ-1000 with rekordbox in performance mode, that roll is done within the software, not the hardware, so you're unable to "move" the roll to another channel as you can when in hardware more (or with a DJM mixer).
There's also a catch even when doing it with a line input... if you had CH3 on a line in, but CH1 is assigned to rekordbox, you could sample the roll on CH3, but if you change the FX assign channel to CH1, it would convert to software and drop the roll. If CH1 was also assigned line-in, you could move the roll there, but any time you move "past" a software channel, the FX are then assigned to rekordbox, losing the roll sample.