You don’t “extend” a zwave network the way you do a WiFi network. Zwave is by specification a low-power network. A message is sent the distance of one “hop” and then passed along by that device for another hop and so on (up to four hops from the hub) until it reaches the intended destination. So you make the network stronger by having more devices available to pass around messages. Not by increasing signal strength. It’s likely that all devices are already operating at maximum signal strength.
Only mains-powered devices can act as repeaters in this way. Not battery powered devices.
Single purpose repeaters (sometimes called range extenders, although that’s a pretty misleading name) were useful up until Zwave generation three. But we are now at generation five, and for the last two generations pretty much any mains- powered device, including light switches, outlets, pocket sockets, light bulbs, and relays, all do their main function and also act as repeaters. So there’s just no reason to buy a single purpose repeater. It won’t boost signal strength any higher than what your other devices are already using, and there’s no reason to use up a hop for a device that does nothing but repeat.
So to improve coverage for your Z wave network, just add more dual purpose devices which are also repeaters. 
http://thingsthataresmart.wiki/index.php?title=Repeaters
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