I’m running Retropie 3.5 and have transferred some Infocom games in .z3 format to the zmachine folder. They’re not being recognised by emulationstation, though the default 3 Zork games (which are in .dat format) are?
I added the extensions as well, so in the future they’ll show. if you don’t want to rename you can also add it to your es_systems.cfg or update you setup script and reinstall frotz to get the most recent changes.