Draw the part you want into the sequencer, set the L and R locators either side of it so when you press play it just plays your part (make sure its solo'd if youve got other parts playing) and then in the file menu theres an export option.
Open an NN19 and press the open file button in the middle of the sampler, not the one on the left as that will only open NN19 instrument files, and choose the WAV you've just been messing with. Trigger it in the piano roll or with your midi keyboard.