Traslation rule from iptables to mikrotik ROS

Hello,
I have five NIC:
NIC 1 = internal lan (192.168.1.x)
NIC 2 = external lan ( internet 1 )
NIC 3 = external lan ( internet 2 )
NIC 4 = external lan ( internet 3 )
NIC 5 = external lan ( internet 4 )

All clients are in different srclist ( 1, 2, 3, 4 ), every srclist use a relative external lan.

srclist1 use an external squid proxy server ( configured in ROS as parent proxy ): it works fine.
Now I wish use a proxy server for others external lans.

In ROS I can set only a parent web proxy server. To use more proxies I think to write some firewall rules, like this:

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to 8080

how can I do it on ROS ?

Do You have a better solution ?

Thank You in advance.

ip firewall nat add action=dst-nat chain=dstnat dst-port=80 protocol=tcp to-ports=8080 to-addresses=address_of_proxy_server

http://www.mikrotik.com/testdocs/ros/2.9/ip/nat.php