How to construct your own drumkit in Groove Agent SE4?

I’ve searched the forums and the owner’s manual for information regarding this but haven’t come up with anything useful. I have a few drum sample collections downloaded from the internet that I’d like to construct a kit(s) with. Dragging the sound files onto an empty pad and saving it as a new kit is okay, but how do you manage things like velocity layers (that I’m guessing require separate sound files to implement)? Any advice and/or links would be appreciated.
Also, what’s your opinion about Groove Agent SE 4 versus other drum programs like EZ Drummer, etc.?