You should be able to write your own smart search index ... since you would be able to set how the content is consumed, you should be able to inject non-Kentico content through it. The biggest obstacle will be how to get that content to be crawled. Since Smart Search isn't really a page scraper, you would probably need to load up each page and parse it out to include into the index.
Here is info on customizing it: https://docs.kentico.com/display/K8/Creating+custom+smart+search+indexes
The other option could be do run your own search with an engine such as Azure Search. Bryan Soltis just wrote an article on that here: http://devnet.kentico.com/articles/integrating-azure-search-with-kentico
I am actually doing my own search through that for a client right now as well. With this you can set multiple indexes from multiple sources easier than having to code your own...but you will need to code your own search interface...so it just depends on where you want your work to go.