We’ve never looked into IdentityServer so we have no idea how much effort would be required. Recently @tobi was looking into integrating Identity Server’s JWT support with ServiceStack’s JWT Auth Provider, so maybe he as a better idea on how feasible it is.
The other approach is to completely ignore/bypass ServiceStack’s built-in Authentication and just use what IdentityServer gives you as it goes through the same Middleware / Request Pipeline in .NET Core so may be a potential solution.