@mcf57 

Range Extenders will use the Wifi Credentials of the network they are extending.  There is no way to "customize" and give an extender its own credentials.  The best you can do is set a seperate SSID.  However your client devices are still connecting through the source so typically would be visible to any device on that network.  Also a range Extender must be placed in range of a Source and provided a good signal from that source in order to provide a good signal to client devices.  

The way you have described your intended setup would not be possible with a range extender.