I don't have one of these to test them, but according to the manual - you should be able to plug in a USB drive (optical or flash) and install software to your PXI-8840.
PXI-8840 User Manual - on page 13 it references how to install "other software"
I also found this for the next model up running Linux, the PXI-8880. Running Linux you can install new software (including Python interpreter) from their repository.
PXI-8880 Linux RT controller adding software - This page also instructs you on how to install the Linux image on your RT controller. (Again YMMV, not sure this is 100% compatible with the 8840)
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