Unsurprisingly, you can enter text into Notepad++ using the keyboard, or an IME window for Far East languages with an ideographic script. Windows provides a way to enter any character in the 0..255 range using the numeric keypad - hold Alt and type0+the character code. Provided you enable the facility, you can also use hexadecimal digits so as to enter 16-bit character codes in a similar way.
Scintilla will use the active font to display the character being entered, so the font must be attuned to the intended rendering of the character. A square or ? usually marks a character the font doesn't know about. Note that fonts may support multiple character sets, but there is no obvious way to specify this to Notepad++.
Notepad++ provides a handy way to enter any character in the 0..255 range, that is Edit -> Character panel.... Double click on any of the 256 rows of the list to insert corresponding character. The list window is dockable for more convenience. Each row shows decimal, hex and glyph/symbolic name for the character.
Newer versions of Scintilla provide Ctrl-Shift-<letter> shortcuts for control characters: for instance, Ctrl-Shift-G enters a BEL (0x07) character. Note that these shortcuts do not work if the letter needs an extra modifier, like @ or \ on various non-US keyboards.