Customizing Captions using NGG Custom Fields

I might be trying the impossible based on the reading I’ve done so far and it is OK to say so. I have created several custom fields that I’ve used to caption my photos, and those appear as I want them to under the photos. But in the lightbox that appears if one enlarges a photo, I have yet to find a way to incorporate the custom field data into the caption that appears there. Is there any way to change this, or is the only option to fill out the alt/title fields manually for each photo?

https://orenstransitpage.com/wordpress/grand-central/united-states/new-york/ithaca/tcat-new-flyer-d40lfs/

Thanks!