As stated in the official document, it is possible to enable android multiple user feature at build time, but there is no document about enabling it on an exiting android image. Is there a way to enable multi-user on an existing device (e.g. using adb) considering that root access is available?
One can edit /system/build.prop on a rooted device and add multi-user support, then reboot:
fw.max_users=3
fw.show_multiuserui=1