I am not sure if KX-TDE600 has a license for third-party SIP devices. Please check and confirm but this (KX-NCS4716) looks like it.
On KX-NS500, i have deployed Analog phones through ATA at remote sites using KX-NSM7XX license. However, you will not be able to use KX-TDE620 remotely as it has to be physically connected KX-TDE600.
See DAG2000-16S (192.168.0.105)in attached diagram, registers to KX-NS500 (192.168.0.101), but the analog extensions from DAG2000-16S are in a remote building registered to KX-NS500 over fiber.
You could use Yeastar ATA but i found Dinstar to be cheaper.
Perhaps someone can confirm if KX-NCS47XX supports third-party SIP devices. I haven't tested on KX-TDE series but i have tested and confirmed with KX-NS300, KX-NS500, KX-NS700 and KX-NS1000 IP-PABX
The choice of your ATA will depend on how many analog extensions you want. The other option is to use IP Phones which is expensive offcourse.