Is there any way of using an external private nuget feed (either an Artifact feed belonging to another organization, or a standalone private (basic auth) enabled nugetserver) as an upstream source in Azure Artifacts?
It is possible in other parts of Azure Devops as a service connection, but I can't find a way of either utilizing service connections nor specify a private external nuget as upstream source in Artifacts. The only options are public nuget servers and feeds present in Devops.
Not yet. We're considering building support for custom upstream sources next quarter. This Developer Community suggestion tracks the idea.