compatibility

can airport time capsule be used as wifi extender with Starlink router

Yes and No.

Yes......if you connect the Time Capsule to the Starlink router using a permanent wired Ethernet cable connection and then configure the Time Capsule to provide a Wi-Fi network that uses the exact same wireless network name and password as the Starlink router.

Post back if you need more tips on this type of setup.

No......if you are asking if the Time Capsule can connect to the Starlink network using only a Wi-Fi connection and extend the network in that manner.