Enable external FTP Access

Hi

I'm running Firmware:378.54_2 on rt-a668u

Is it possible to remove the admin access or is this admin user restricted to the internal LAN ?
I want to enable external access and allow 3 unique users to have access to specific folders on a USB disk.

Any guides how to do this ? The Asus doc I have doesn't look like like the FTP section on my router.

Thanks