FtpServer="192.168.0.2" tcp_allow="{ 22, 7778 }"
rdr on $ExtIF proto tcp from any to any port 7777 -> $FtpServer
pass in on $ExtIF inet proto tcp from any to $ExtIF port $tcp_allow \ flags S/SA modulate state
In addition to what Alexey said, read this:
http://www.openbsd.org/faq/pf/rdr.html#filter
Note also that $tcp_allow specifies port 7778 while your rdr rule specifies 7777.
.joel