Is it possible to edit or add an extra coded value to uploaded domains associated with a feature service on ArcGIS online?
Yes.
It is easy to change values - a little trickier to add new values. Here is an explanation:
https://blogs.esri.com/esri/arcgis/files/2014/10/How-to-Update-Hosted-Feature-Service-Schemas1.pdf
You could also read this:
Is it possible to edit domain coded values in ArcGIS Online Feature Layers? - Geographic Information...
Appreciate the info and link Sveaskog, I accomplished it using the "longhand" approach as I was in a hurry for a solution but will keep this for future reference.
