kidkoThe nozzles on the NE800M's are quite a bit larger than the Etymotic line. Pretty sure those won't fit unless you somehow stretched out the tip a lot to force it on. However, that would likely degrade the sound quality and sound stage as the bore of that tip is too narrow for how the NE800M is designed.
http://www.amazon.com/Etymotic-Research-ER38-15SM-3-Flange-Replacement/dp/B0078JXDQ4/