Workshop support
FoliesApp and the Steam store description both state that players can build their own sound packs and publish them to Steam Workshop. The supplied SteamDB capture also marks Workshop visibility as enabled.
Sharing with a group
A community pack must be installed by everyone at the table before that lobby can start a game with it. Use the lobby status to see who is still downloading rather than assuming every subscriber is ready.
What is not documented here
The supplied material does not give a step-by-step editor workflow, file format, or moderation policy. Those details should be checked in the live game and Workshop pages when available.