I had to Google this, so I figured I'd throw up a quick how-to in case it can save someone else some time.
I like the Metal Foundry kit, but I don't like any of the snares that it's bundled with. DKFH has a nice tight snare, so I wanted to try that out to see how it would sound in the kit. This is what XDrum does - you create a new drum, and map it to the MIDI note of the existing snare in the kit.
So here's Metal Foundry:
In the top right, under XDrum, make a new drum:
Pick the kit that you want to import the drum from. In this case, I'm using Drumkit from Hell.
Once you pick the kit, pick the instrument. The new XDrum will show up in the top right corner.
Then go to the Mapping tab, and check the piano roll for the MIDI note assignment of the existing snare:
Mouse over it, it's 38. So set the new XDrum assignment to... You guessed it.. 38. (Or whatever the MIDI note is that you're replacing).
Voila! Go back to your kit, and any MIDI triggers you send into it will now use your new XDrum instead of the default piece.
Won't you have to remove the mapping on the existing snare? Otherwise it's triggering both at the same time.