Ita easy to do in unraid, you set it up per share, so say you have a "media" share you can change the settings to include a cache drive and then set it to write to the cache drive first and then more to array. If you don't have a cache drive or want to add a other you can do that by installing the ssd, booting up, stopping the array and adding in a new cache drive (you can add it to your existing cache pool to increase its size or create a new one and keep them separate for separate uses)