Tattie: ABC is mostly just a straight-text way of representing standard notation--very simple to learn. So if you've got a score, either on manuscript paper or in Notepad, it should be straightforward to jot down the equivalent ABC notation. An ABC tool like Barfly, which allows you to view the ABC in standard notation as you write it, can help verify the transcription.

Finale Notepad (like Finale and PrintMusic) may support exporting as MusicXML, which might be importable to an ABC program; see my caveat above. Similarly, some music editors can reconstruct scores from scans of printed music, which can then be exported as MusicXML, etc.

FWIW, MusicXML is itself a purely text-based representation, but so verbose that it's not amenable for either posting or for simple creation by hand.