Zigbee only extends zigbee, Z wave only extends Z wave.
Start with this FAQ, and in particular, read post 11. It should help a lot.
After reading that post, just scroll up and read the whole thread, there’s a lot of helpful information there.
And if you want even more information after that, see the article on repeaters in the community – created wiki. 
http://thingsthataresmart.wiki/index.php?title=Is_this_device_a_repeater%3F