If you can write using the numeric keypad, that means you already have the code point for the character. Simply enter the hex value of the character(s) you want into Notepad++, select them, then open the menu: Plugins → Converter → HEX -> ASCII
Demonstration:
ASCII to HEX and HEX to ASCII Conversion Notepad++
You can also get live results from the conversion panel
Another way is using the HexEditor plugin, which was included in prior versions of Notepad++ but was removed recently. You can still install the plugin (or reactivate it if it was disabled) from the plugin manager with the caveat that sometimes it may be unstable.
This way you just select Plugins → Hex-Editor → View in HEX (or click the H button near the right of the menu bar) then type the UTF-8 bytes into the hex edit window. For example to get the string 🔙🔚🔛🔜 which is f0 9f 94 99 f0 9f 94 9a f0 9f 94 9b f0 9f 94 9c in UTF-8, just type those hex values in to dump column and switch back to normal text mode; you'll see those characters appear.
Note that it might be easier to work in UTF-16 or UTF-32 since getting UTF-8 encoding of a character manually is tricky. After all, just convert the file back to UTF-8 when saving
It's also possible to use the Base64 converter for this purpose. Just select the base64 encoded string and select Plugins → MIME Tools → Base64 Decode
If you just want to enter a few special characters frequently, it's better to use a macro.
First you need to get the base64 encoding of the string by pasting it to Notepad++ and then use the Base64 Encode feature. After that select Macro → Start recording, type the base64 string you get, select it and decode it as above. Now you can stop recording and save the macro with some descriptive name and possibly along with a shortcut. It's also possible to use the HEX → ASCII feature instead of base64
Later when you want to insert that string, just replay the macro
#Edit:
Edit:
The Conversion Panel works with Unicode only if you paste a Unicode character directly in the ASCII field
