As of EmberZNet 6.0.0, we don't have EFR 32 sniffer functionality integrated into Simplicity Studio yet.
However you can use direct capture, an EM357 sniffer, or build your own sniffer image:

1. You can always make a direct capture in Simplicity Studio Network Analyzer from any EM35x or EFR32MG device provided that you have access to the packet trace interface. Simplicity Studio's Network Analyzer debugging tool provides a rich and flexible interface to Silicon Labs embedded networks, which helps you develop and debug.

2. Use EM357 as sniffer through ISA3 debug adapter. You can view the ISA3 debug adapter device in Simplicity Studio, then make the connected EM357 as sniffer by right-clicking and selecting "make sniffer".

3. Follow our KBA on Turning any EFR32 into a Zigbee or Thread Sniffer.