NaturalVoiceSAPIAdapter is a free, open-source project on GitHub that registers natural voices as ordinary SAPI 5 voices, so programs like Balabolka can pick them. It can use Narrator's installed voices (offline), Edge's online voices, or Azure voices with your own key.
Installing it
Download the release zip, extract it to a folder you'll keep, run Installer.exe as administrator, and install both the 32-bit and the 64-bit versions, because 32-bit apps such as Balabolka only see 32-bit voices. Then choose the voice sources.
The catch
It works by using encryption keys taken from Windows' own files. Its author calls it more of a hack than a proper solution: Microsoft doesn't allow other apps to use these voices, and an update can break it at any time. On Windows 11, the latest Narrator voice versions reportedly don't work with it, and its wiki points to older ones. If Narrator itself stops working, removing the voice packs is the suggested fix.
Fine for an experiment. We wouldn't build a daily workflow on it.