It seems something like this would work
Code:
from calibre.ebooks.conversion.plugins.pdf_input import PDFInput
class MyPDFInput(PDFInput):
name = "MyPDFInput"
description = "Adds support for additional file extensions to PDF input"
supported_extensions = ['pdf_original', 'pdf_answers', 'pdf_booklet', 'pdf_oa', 'pdf_ocr', 'pdf_old', 'pdf_original', 'pdf_tesseract']
But I don't know how to test it.