Could you draw your net? Because from what you have posted, it seems that you do not need static routes, but just use an alias ip on the lan interface of the ZyWALL, but I think you haven't posted something vital
Anyway if all traffic goes through a router to get to the second net, then this is what asymmetrical routing block was designed for. Ie not allow any other device to be used as a gw out of the network.
If you do not need to go through a router to get to the second subnet, then an alias on the ZyWALL will do.