Mikrotik have the option to define navigation ip ..
Please if you can give me an instructive
If u want to set IP address when your mikrotik is doing src-nat u need to use a src-nat rule and not a masquerade one.
Use something like this:
/ip firewall nat chain=srcnat out-interface= src-address= action=src-nat to-address=
You must have IP address configured at out interface.
Is it what you need? if not please send more information.
Jorge