Using Wlan for output traffic for the wired clients

Please excuse me if I say something very wrong, as my english is not perfect and actually I am not a networking expert…

This is my scenario, and I cannot figure out how to configure:

1.- I have a mikrotik that receive in the interface 1 the traffic from a DSL; to do this I could configure as bridged and PPPoE ; this is working fine
2.- I have a wifi connection configured, so the mikrotik can connect to some Wifi connection as client, and this seems to be working ok;
3.- I have a wired client connected in interface 2; this client is a raspberry Pi with an squid proxy, and I have a NAT rule so traffic can reach the raspberry coming from Internet; this is working fine too.

And now this is my problem: I want that the squid use the wlan for outgoing traffic this is:

some client (http) ==> DSL ==> squid proxy ==> wlan connection to Internet

4- What I want is use the IP provided by the Wlan network but use the DSL for receiving, as the DSL is a fixed IP.

I could configure the 1,2,3 steps but I can not find the configuration for the 4th step

Thanks a lot!