After searching some more, I think it is possible.

Have a look: https://www.youtube.com/watch?v=mIp6u0xFPdg

But it is also about how the drum tracks are written. It seems Yamaha goes more conservative, more "carefully" organized, individual drums sounds being balanced for a more uniform level, which I think gives it that compressed sounds. Korg is more loose in a sense - just like the real band. Well, you know what I mean.

The change of samples will change Yamaha's overall sound quite nicely, so I think it's a good idea to try it. It seems not that many players feel like this, so not a ton of demos out there, but I like what was possible with custom samples inclusion. And the software to use for this seems to be good and easily mastered.

I am now just curious if someone here had the experience of adding custom samples to their S770.