Following Robert Hatcher's and Herilala Razafinime's suggestions.
Someone should use "dk2nu" flux files in order to create off-axis samples, as the "gsimple" flux files already have the beam-to-detector coordinate "baked in," making the modified "GNuMIFlux.xml" file not seen.
Solution:
Using the command (for the creation of a 30m off-axis sample):
python ProcessND.py --pot 1e14 --outdir /pnfs/dune/scratch/users/your/path --topvol volDetEnclosure --oa 30 --use_dk2nu
and then manually include the line setup dk2nugenie v01_10_01c -q e20:prof in the processnd.sh