Figured out how to do it indirectly.
In \retropie\configs\all\emulationstation\es_systems.cfg
<system>
<name>mame-mame4all</name>
<fullname>Multiple Arcade Machine Emulator</fullname>
<path>/home/pi/RetroPie/roms/mame-mame4all/roms</path>
<extension>.zip .ZIP</extension>
<command>/opt/retropie/supplementary/runcommand/runcommand.sh 0 SYS mame-mame4all %ROM%</command>
<platform>arcade</platform>
<theme>mame-mame4all</theme>
</system>
In \retropie\configs\mame-mame4all\mame.cfg
rompath=/home/pi/RetroPie/roms/mame-mame4all
samplepath=/home/pi/RetroPie/roms/mame-mame4all/samples
Put all game zips in /home/pi/RetroPie/roms/mame-mame4all/
Duplicate empty zero byte zip file names under /home/pi/RetroPie/roms/mame-mame4all/roms to generate the menu in subdirs.
This also fixes issue where sample.zips shows up in emulation station menu.