Bringing aerial cable into an existing PVC Conduit Body
tl;dr I wasn't able to achieve the install without another hole, but it all worked out cleanly in the end.

Thanks to all who commented on this, your insight was helpful as I thought through how to do this. Ultimately, I wasn't able to find an actual port cover or extension ring anywhere -- seems like a potential business opportunity, but maybe this is a rare scenario :)

Today was install day, and ultimately it was quite successful. Since a slackbox was required to hold the excess fiber, and combine the outdoor and indoor cables, the tech drilled a 1/4" hole through the back of the interior junction box to exit the house to the right of the conduit body. The fiber enters the back of the slackbox and it's a nice clean look. It's sealed up with silicone, and just needs a coat of paint to make it blend in to the house. The aerial fiber comes from the pole and down behind the downspout.



On the inside, we attached pull string to one coax line in the wiring cabinet and pulled it into the junction box. Then, we attached the fiber to the coax (along with another pull string for the future), and re-pulled it back to the destination. It's basically a straight run along an interior wall, and worked out great. The ENT is 1.5" diameter, I believe, to answer a prior comment. It's got a bunch of wires in there, but there was plenty of room for the fiber. And yes, the interior cable they used is pretty bendable, so had no issues in that regard.