We've added the community made skins to the GT3 pack in the latest update. For release notes, please read this forum post:
https://forum.studio-397.com/index.php?threads/latest-release-1109-rain-effects-and-more.58675/page-38#post-939998
Also, we have updated the two karts that are part of the KartSim pack. Release notes for that one are here:
https://forum.studio-397.com/index.php?threads/introducing-kartsim.59673/page-9#post-939999