PaulS:
OP wants to use an external editor, just like you do. Only he/she wants to use Notepad++ instead of Geary. At least, that what I think he/she wants. Looks like English is not his/her native language.
Sp Geany
I have not used Notepad++ but AFAIK 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. When you click Verify or Upload it uses the latest version of the edited file.
Geany works with Windows as well as Linux.
...R