It is not clear to me what you are trying to achieve. So you have site A and site B, with different network infrastructure/subnets/VLANs. Do you want to replicate/configure the same network/VLANs/subnets of site B behind the FortiGate of site A, so you can then move one (or more) device and test? Or do you want to achieve that using the VPN you have between the two sites? If yes, is it a site-to-site IPsec VPN?
In that case I think that you can configure the FortiGate on site A from GUI or from CLI. Please bear in mind that, based on the complexity of your configuration, some planning/design considerations may be needed. If you are going to have the same subnets on both sides of the IPsec VPN, you may need to configure routing properly to avoid any unexpected issues.
caveat: when you create interfaces for site B Subnets on Site A this also creates static (connected) routing which will mean that you can no longer route that to site B!
So might be better to use some lab environment that is behind NAT or similar to avoid this.
