If this is mainly for the device, you can use a metadata plugboard to fiddle with the author when it is being sent to the device. I assume that AO3 is just an example and that you want to add the source of the fanfiction as the first author. If the source site was in a separate column, then you could at it to the start of the authors when sending the book.
I'm not exectly sure what a Kindle does with this, but setting the author using the following is a starting point:
Code:
{#fandom_source} & {author}
You would need to create the column #fandom_source and populate it. If you are using FFF to download them, it shouldn't be to hard.