Yes, you could do that with address lists. They even take ranges. See below:
/ip firewall address-list
add list=pool1 address=192.168.0.2-192.168.0.6
add list=pool2 address=192.168.0.7-192.168.0.11
add list=pool3 address=192.168.0.12-192.168.0.16
add list=pool4 address=192.168.0.17-192.168.0.21
/ip address
add address=1.1.1.2 interface=WAN
add address=1.1.1.3 interface=WAN
add address=1.1.1.4 interface=WAN
add address=1.1.1.5 interface=WAN
/ip firewall nat
add chain=srcnat out-interface=WAN src-address-list=pool1 action=src-nat to-address=1.1.1.2
add chain=srcnat out-interface=WAN src-address-list=pool2 action=src-nat to-address=1.1.1.3
add chain=srcnat out-interface=WAN src-address-list=pool3 action=src-nat to-address=1.1.1.4
add chain=srcnat out-interface=WAN src-address-list=pool4 action=src-nat to-address=1.1.1.5
Another interesting approach - though I have never tried it - is to use PCC to balance clients across multiple addresses. The more clients, the more even the balance will be. You could pick other classifiers such as both-addresses-and-ports, but that would make secure sites rather unstable as they often rely on the client to come from the same source IP for the duration of a session:
/ip address
add address=1.1.1.2 interface=WAN
add address=1.1.1.3 interface=WAN
add address=1.1.1.4 interface=WAN
add address=1.1.1.5 interface=WAN
/ip firewall nat
add chain=srcnat out-interface=WAN per-connection-classifier=src-address:0/4 action=src-nat to-address=1.1.1.2
add chain=srcnat out-interface=WAN per-connection-classifier=src-address:1/4 action=src-nat to-address=1.1.1.3
add chain=srcnat out-interface=WAN per-connection-classifier=src-address:2/4 action=src-nat to-address=1.1.1.4
add chain=srcnat out-interface=WAN per-connection-classifier=src-address:3/4 action=src-nat to-address=1.1.1.5
add chain=srcnat out-interface=WAN action=src-nat to-address=1.1.1.1.2 comment="paranoid fallback"