ftp-proxy

In the base system, load the daemon if you need it

/etc/rc.conf[.local]

ftpproxy_flags=""

/etc/pf.conf

NAT section anchors

nat-anchor "ftp-proxy/*"
rdr-anchor "ftp-proxy/*"

the redirection

rdr pass on $int_if proto tcp from any to any port ftp -> 127.0.0.1 \
         port 8021

in your filtering section

anchor "ftp-proxy/*"

pass out proto tcp from $proxy to any port 21 keep state