using an external editor alongside the IDE is as simple checking the box in Preferences and then using your preferred editor to edit the .ino file.
Yep. I'm surprisingly pleased with the way that works...
The original post is NOT about customizing the Arduino IDE to use NPP as an external editor (which is easy, as described by Robin2) - it's about customizing Notepad++ to recognize all the arduino keywords and stuff. If that's too tough, complain to the Notepad people (but ... fancy editors tend to require complex configuration to utilize all their features. At least they don't seem to require you to learn LisP. 
)
(The original message is also from 2013. I'd be surprised if the rising popularity of Arduino hasn't resulted in an easier customization process for NPP. Um, maybe N++ Arduino Plugin download | SourceForge.net ?)