If you’re a Ghost Pro customer (hosted on ghost.io), you could contact them directly to ask if they can add ads.txt for you. Since the starter plan doesn’t allow you to upload a theme, I’m not sure there’s a workaround anyone on the forum can offer you.
Hang on… what about this? Looks like you can do a single redirect to an ads.txt file hosted elsewhere…
Source: Ads.txt FAQs - Google AdSense Help
I’m pretty sure the starter plan allows you to upload a redirects file, so this might be a good workaround?