The best i can recommend is using one Groove Agent instance per sound (eg. for a snare sound, including possible samples for layering).
You put your sample on the “C3” pad and activate the mode, where you can play the sample across the keyboard range, if needed. Additional layers of samples can be added to the same pad, including ADSR and such.
You have to browse for the sample in Media Bay/Finder/Explorer, though (or drag them from the project, if already placed on a track). There’s a browser integrated into the full version of Groove Agent, which also features hot swap. The basic version that ships with cubase features hot swap via Finder/Explorer load files dialogue.
You have to judge yourself if the full version is worth it for you.
Personally, i’m using Vengeance Phalanx, now. It’s easier to use and more powerful imo, so i would look into that if the included Groove Agent doesn’t do it for you. There might also be a new “simple” sampler coming up in Cubase 9, but that is speculation.
Best,
Tim