I wanted to add a comment in support of this feature.
I wrote a workflow that returns folders and files that I often access, and having to remember to use the workflow keyword is difficult, and I would rather have the results of that workflow just be in the default results.
I've read a few other places in the forms where it's been mentioned that it isn't possible to add workflow results to the default search results because it could slow down the default results.
A few remarks:
1. There are slow workflows that could slow down the search results, but there are many workflows that would be useful to include in the default search results that are very fast, and wouldn't slow down the results at all. I added a workflow that returns commonly used folders and files, and it takes less than 1/100th of a second to run.
2. The risk can easily be remedied by including a warning that adding a workflow could make the default search results slower, so that users can be aware of the risk and decide whether or not it's worth it for them.
3. Workflows can be run asynchronously, in another thread, and so wouldn't have to slow down the main search results.
4. This feature has been requested a number of times, aside from this thread, so it seems that it's a common desire: