How do I collect return values from Parallel.ForEach?

You’ve discarded it in here.

ParallelLoopResult result = Parallel.ForEach(words, word => AddB(word));

You probably want something like,

ParallelLoopResult result = Parallel.ForEach(words, word =>
{
    string result = AddB(word);
    // do something with result
});

If you want some sort of collection at the end of this, consider using one of the collections under System.Collections.Concurrent, like ConcurrentBag

ConcurrentBag<string> resultCollection = new ConcurrentBag<string>();
ParallelLoopResult result = Parallel.ForEach(words, word =>
{
    resultCollection.Add(AddB(word));
});

// Do something with the result