When C3 imports a C2 project, it guesses the property IDs by transforming the property name. The easiest thing to do is choose IDs in C3 that match the ones it guesses. I think it will guess "script-files". If for any reason you can't do that or you need to change the type of data it imports, here's an example from Tiled Background that converts an "initial visibility" property to a boolean with the new ID "initially-visible":
LoadC2Property(name, valueString)
{
switch (name) {
case "initial-visibility": // renamed, and map Visible/Invisible string to boolean
this._inst.SetPropertyValue("initially-visible", (valueString !== "Invisible"));
return true;
}
}