Hi Marek,
on EXOS, IGMP snooping is enabled by default, you may not need additional configuration.
I have not tested DHCP snooping on EXOS, so I cannot comment on that part.
When adding a switch to NAC (ExtremeControl), NAC can be used to configure the RADIUS servers. After the switch is added, NAC can be used to configure authentication on the switch. This needs a working SNMP configuration.
You can add the RADIUS server configuration manually, if you prefer that.
Erik