Can I use notepad++ for writing sketches?
You can easily use notepad++ or any text editor to edit arduino code files (.pde or .ino). I use the sublime text editor for all my arduino coding.

As you noted, the arduino syntax is c++, so setting the syntax highlighting mode to c++ is all that you need to do; no mod should be necessary.

Arduino includes a "Use external editor" option in the preferences menu that will make it reload the file every time you compile or upload. It also disables editing in the editor window. In this mode, just save your file in any editor, change to the arduino window, and hit upload. 

Since 1.5, arduino has had command line support. most editors allow making the "build" button execute arbitrary commands in the current directory. by setting build to run "arduino --verify" or "arduino --upload" you can code in any editor you want even without having an arduino window open.