SteamOS is currently supported as part of the Linux target. You will need to use the GitHub source, along with the optional zip and the Linux cross-compiler toolchain to enable Linux support.
EDIT: I would also recommend against putting a Linux UE4 game up on a store for now. Current Linux support is very very rough, and there are several issues (sound doesn’t work, binaries are not automatically created w/ the executable bit, etc.) that we are working to resolve for future updates…