opkg update && opkg install dropbear-ssh
This package uses passwd/shadow files installed in /mod/etc/dropbear/. The only user that's defined when you first install the package is root with a password of humax but you can change the password or add new users by hand and subsequent package upgrades won't overwrite it. That directory is also where the server keys are placed - they are generated the first time you install the package.
SCP's supported but SFTP isn't just yet. SFTP will be a separate package coming shortly.