Hi Bhushan,
There is a very good book on PowerNSX. You can find it on below link:
https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/products/nsx/vmware-automating-vsp...
In response to your questions:
You can use cmdlet New-NsxFirewallSection to create new firewall section. Page ref. 92
Also, you can use below to add IP address to existing IPSet, Page ref. 95
Get-NsxIpSet -name IPS-RFC1918 | AddNsxIpSetMember -IPAddress “192.168.0.0/16”
Let me know if it helps.