You can use recycle to chop up the sample you wanna use then use the Dr.Rex and open the .rx2 file you saved in Recycle.
For the NN-XT well it's simple too! You load a wav sample you want, once that's done, you will see in the big blueish windows the sample you loaded in and it is placed below a piano key. That key is generally the one that will have the sample on it. So if you press that key well the sample will play. Now if you want the same sample to play on like per example Key C2 too C5 well click on the sample in the window, it will higlight it and you will be able to enlarge the sample key to C5. Now if you press on every key from C2 too C5 you will hear the same sample but at a different speed at each note. If you want to load another sample in the NN-XT and keep the one you used earlier, well you do the same thing as for the beginning by pressing the "load sample" button.
I don't know if this is clear for ya but if you don't understand, well when I'm gonna go home after works I'll screenshot you the method!! By then well ask me a question and I'll try too help you out!!
P'z
ZonQ