Extend an Apple wireless network with non-Apple gear
Unfortunately Apple based networks cannot be extended using non-Apple devices. Even though in theory it should work without any problems, Apple has restricted using 3rd party routers to extended wireless network, and vice versa, non-Apple based wireless networks cannot be extended using Apple routers.

The workaround is to connect both, your Airport Express and your Netgear using Ethernet cable and enabling bridge mode in your Netgear settings page. Bridge mode disables creating second sub-network in your LAN, making Airport Express your default DHCP server.