NAT to internal/Local Firewall IP.

Hello, I am configuring a Fortigate which has a private IP in WAN interface. I need to create S2S IPSEC tunnel but I don not have a local public IP. Is there any possible to create a NAT in the firewall to MAP the local WAN private IP to a public IP (the ISP is sending the Public IP traffic to the Fortigate through routing)? Thank you very much.

Best Regards.