Great question. If you have full rights, permissions and licenses to the author's work, you can include documentations, such as a legal statement or affidavit from their estate acknowledging your rights and the intended use of their voice.
In order for Custom Neural Voice to recognize them, Microsoft requires documentation verifying your authorization or ownership of rights for these external sources. Since every case is different, you might want to submit your documentation or contact Microsoft directly for further guidance on your particular scenario.