To specify a port range for DCC:

/set dcc_port 6000 7000

Make sure your NAT router has DMZ or port forwarding for the nominated ports and that your firewall has those ports open.

Your config file will look like this:

jj5@honesty:~/.irssi$ grep -R 6000 .
./config:  "irc/dcc" = { dcc_port = "6000 7000"; };