Generally a good "real" instrument ReFill will be set up to play only the range of the instrument (or what will work anyway).
This is not too hard to change as the Key Range or Key Zone of an instrument is right there in NN-XT above the keyboard in the LCD screen. Find & Click on the lowest sample in the patch and see where there is a range shown with grab handles above the little keyboard. Drag the lowest lower and now the instrument will play below (I'm assuming) E. Not quite 'drop' tuning but will get you through.
You may need to consider changing you voicing however if you are emulating real world performances as you have just crossed that line.