The simplest way is… to literally just assign the addresses to your "WAN" interfaces in exactly the same way as you've currently assigned the 192.168.1.x.
Decide on whatever subnet size you prefer (let's say /24), then assign 126.96.36.199/24 to the Kali VM and another address from the same subnet (for example 188.8.131.52) to the pfSense WAN interface.
The reason you're not finding any information is because there is nothing to simulate. On the technical level, there is no difference between "public" and "private" IP addresses – they both are configured in the same way, routed in the same way, subnetted in the same way, and so on.
The only difference is just policy: real-world RIRs will refuse to issue private addresses, real-world ISPs will refuse to accept private routes on the public Internet. But since you're on an isolated network you can do anything you wish, and that includes just straight up configuring any address on any VM.