You don't say exactly what make & model of "extender" or if they are dual band etc.
If you have a setup with multiple WiFi access points, whether stand-alone, in a router or in an extender, you can assign the same network name (ssid), security mode and password to all of them.
They should each be on different channels.
Any WiFi device should then roam between them, using whichever AP is providing the best signal at any location.
That's easy with ethernet-connected APs (or ones connected by homeplug adapters) and should be possible with dual-band ones, where the uplink is on a different band to the client AP.
Single band wifi-connected extenders sometimes cannot use the same SSID as the main network.
Note that any extender that links via WiFi to the main network is a bad idea and should only ever be a last resort - everything has to be transmitted twice, slowing everything down even more and making congestion and "hidden station" problems far worse. Use Ethernet or Homeplug feeds if at all possible.