Hi @bryan-ellis_2367 ,
I'm not an Azure DevOps expert, but last I checked, it's not possible to add NuGet package sources other than it's own ADO Packages product or the public repositories. That may just refer to "upstream sources", but I'm not totally sure.
However, if you want to use the ADO Pipeline's built-in NuGet commands to publish packages, I guess you can set up a service connection using this?
https://docs.microsoft.com/en-us/azure/devops/pipelines/library/service-endpoints?view=azure-devops&tabs=yaml#nuget-service-connection
Not totally sure -- but please let us know what you find :)
Cheers,
Alana