I have a similar setup to yours, except for the cisco router. in my setup i have 1 main router, ZS, and 2 network segments each with a ZS. you should not have to make any changes to your main cisco router, simply put a static route on the ZS to your main network:
STATIC ROUTES
Destination |Netmask|Type|Metric|Gateway|Interface|State
DEFAULT GATEWAY|0.0.0.0|Net|0|192.168.0.1| |Up
that is all i needed to do for my setup.
obviously change the gateway I use for yours