ISO 639-3 and the Ethnologue aim to list every language. So, if “Nahuatl, Chiconcuautla” is a “language” (see definition at https://www.ethnologue.com/about/problem-language-identification) then someone should request that it be listed by following the procedure at http://www-01.sil.org/iso639-3/submit_changes.asp.
This procedure takes up to a year, so in the meantime you’ll have to enter something in Paratext, so you could use the ISO 639-3 code for the closest related language, which would presumably be Nahuatl, Northern Puebla [ncj] in this case.
Alternatively, you might be able to use a code from the “Private Use Area” (any code between “qaa” and “qtz”), but this could have implications for the Paratext registry—i.e. conflicts arising from two languages from different parts of the world both picking the same PUA code.
Blessings,
LivingField