You can do it either using mangle to add routing mark or by using src-address attribute of src-nat:
/ip firewall nat
add action=src-nat chain=srcnat src-address=192.168.85.2-192.168.85.149 to-addresses=22.66.33.1
add action=src-nat chain=srcnat src-address=192.168.85.150-192.168.85.253 to-addresses=22.66.33.10
add action=src-nat chain=srcnat to-addresses=22.66.33.254
I’m not sure about the last line, but hopefully it’ll catch any other local address (such as router’s own address) while the first two, being more specific and higher on the list, will deal with your client’s addresses.