Is is possible to provide your own mapping functions when using PopulateWith() or ConvertTo()?
I want to use some custom rules about what to map and when.
No, my recommendation is if you need more than the intuitive default behavior than to add a custom extension method.
So any customizations are explicit, debuggable with the full access to C# and not some new limited configuration language needed to be documented and learned that’s defined nowhere near the call-site where it applies.