De um fórum do Notepad ++ :
Settings -> Styler Configurator, then choose the language you wish on the left hand list. You'll notice two file extension boxes in the lower left part of the dialog. One is read-only and has the default extensions associated. The other "User ext." has whatever extensions you add. Don't copy the leading period, and separate multiple extensions with a space, like in xml conf cfg
Adicionado:
Parece que você precisa criar um atalho para o arquivo e usar a seguinte sintaxe:
/(path)/notepad++.exe -l(cpp language) /(path)/(file)