Once you've taken control of a file for import it's up to you to do all the work.
There is no way to hook in other importers either at this time (it just got added to exporters).
Definitely I am not going to (re)write the importer which already exists .
Is there any way to get default importer source from Adobe?
And if you use open source (like FFmpeg or libav) to do all that work you would be in violation of patents and licenseing terms.