I'm not sure if it is possible to include add-on results in the normal search, or even the global search add-on, but for many add-ons you can map a location as either a
custom home item (wiki) or by editing the skin files directly (see
skinning (wiki)). For YouTube I was able to find out a path that I could map a custom home item or
keymap (wiki) to by turning on the
debug log (wiki) and looking at what it said. I took a few guesses and came up with this for YouTube:
For the list of recent searches - plugin://plugin.video.youtube/kodion/search/list/
For the actual search window - plugin://plugin.video.youtube/kodion/search/input/
If you use a skin like Aeon Nox then things are really easy and I don't even have to look at any logs. You can make a custom home item, select the add-on, and then the add-on window you want to use such as the search option. Then you have a search button right there on your home screen that goes directly to YouTube search.