Use different Nuget feed with Porting Assistant for .Net

Porting Assistant for .NET does not have built-in support to configure an alternate NuGet feed

You can create a custom NuGet.config file in your solution folder with the necessary configurations pointing to your internal NuGet feed. Porting Assistant for .NET relies on the dotnet CLI tool to gather NuGet package information. By placing the custom NuGet.config file in your solution folder, the dotnet CLI will use the specified internal NuGet feed when looking for packages.

example NuGet.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <clear />
    <add key="InternalFeed" value="https://path-to-your-internal-nuget-feed/" />
  </packageSources>
</configuration>

https://docs.aws.amazon.com/portingassistant/latest/userguide/what-is-porting-assistant.html

Internal NuGet

Some users have an internal NuGet hosting service, similar to nuget.org, that is accessible only within their local network.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <add key="InternalFeed" value="http://your.internal.feed/nuget" />
  </packageSources>
</configuration>

Make sure to replace InternalFeed with a name that identifies your internal NuGet feed, and http://your.internal.feed/nuget with the URL of your internal NuGet feed.

I hope the below link also helps this.

https://learn.microsoft.com/en-us/nuget/consume-packages/configuring-nuget-behavior