TNG does not have an auto update extension numbers feature.
There is a feature to import extension numbers form a file,
see How do I import extensions from a file into TNG?
You can auto update the user names if the PABX supports this feature.
This will also add the extension number.
Please note that if you first import extension numbers and descriptions it will update this with what is received from the PABX:
TNG (with buffer)/TNG Live:
Can my user names for extensions be auto updated to TNG?
TNG for Asterisk:
TNG for Asterisk and user names for extensions/pin codes