Not that I'm aware of. It would be fairly pointless because the OS isn't designed to be a multiuser system. That's why there is no useradd, userdel, usermod, groupadd, groupdel, etc.
Sorry I don't use entware so I can't comment on that. If you want a multi-user Linux system I think you'd be better served by using something like a Raspberry Pi. I think others have setup a chrooted Debian environment on the router.