Hi!
Just keep adding to the same 'name'. You'll notice that if you delete the last one then the service set will vanish. There is no service set really, it's a service with an extra field with the name of the service set.
For example, these create two serviceset items in the same service set:
./nrcq
http://127.0.0.1:8888/rest add/servicesets -j -d name:'DEV-LINUX' -d svcdesc:'DISK-/' -d template:'svctmpl-lin' -d command:'check_snmp!$_HOSTCOMMUNITY!/!85%!95%!'
./nrcq
http://127.0.0.1:8888/rest add/servicesets -j -d name:'DEV-LINUX' -d svcdesc:'PING' -d template:'svctmpl-lin' -d command:'check_ping!100.0,20%!500.0,60%'
Cheers!
Mark