Thank you for your message.
This seems to be coming from a version 7. Its possible to include your custom field by customizing following file:
This is a system file and it doesn't use portal engine to generate the form. Please note that such customizations are not supported by default and it might get overriden by future hotfix/upgrade. Especially during upgrade to version 8 you might experience some issues which will need to be addressed.
In most cases I would recommend to create completely custom module which is the safest option.
Feel free to let me know if you have further questions.