public static ChannelReader<T> Concat<T>(this ChannelReader<T> reader, 
                                              IEnumerable<ChannelReader<T>> other,
                                              ChannelParallelOptions options)

using Amphisbaena;
using Amphisbaena.Linq;

...

int[] data = new int[] { 1, 2, 3 };

var result = data
  .ToChannelReader()
  .Concat(new int[] { 4, 5},
          Array.Empty<int>(),
          new int[] { 6 }
   );