While it's possible to add a new package source to Visual Studio this is not optimal if there are more than one developer or if a build server is used.
An alternative is to add a .nuget folder and within that folder add a NuGet.config file with the following content:
<?xml version="1.0" encoding="utf-8"?>
<add key="NuGet official package source" value="https://nuget.org/api/v2/" />
<add key="Some other package source" value="some url to the other package source" />
<!-- this tells that all of them are active -->
<add key="All" value="(Aggregate source)" />