Yes, of course. Don’t use an action of “masquerade” but us “src-nat” with a to-address.
http://forum.mikrotik.com/t/forwarding-public-ip-to-specific-nated-private-ips/48228/1
That shows different NAT actions by IP. You can make decisions based on many more things, such as protocol and port. See the IP firewall NAT manual for all the available properties.