Add another netgear wifi extender to me network

If you're router and your extender are only EX versions, I'd probably go with the EX7500/EX770/EX8000. They'll be the same generation and all have the dedicated backhaul that improves latency and throughput. 

the only thing that I'll advise is I usually NEVER recommend running more than 1 extender. Reason why is mesh extenders just connect to a set ssid (wifi name) without anything controlling them. when you have more than 1, they can end up connecting to each other can causing issues. This is especially troublesome if you have sketchy power because sometimes the extenders boot up before the router and that causes the to connect to each other.  

IF you're going to do it, I'd highly recommend going: 

Extender<--------router-------->extender   

Make sure the extenders are far enough apart that they won't connect to each other.  

You're other option instead of another extender, is to pickup a mesh network. They have a router that controls the system so you don't end up with the same issue. If you're fine with AC devices, the older orbi RBK50's and 40 series are pretty rock solid and can be found used for cheap. Or you can go a little newer with the AX Orbi systems. They can be found used/refurbished as well and are pretty reasonable. 

My advise? Spend the extra and switch to an actual mesh system. You'll have better stability and roaming.