Routing between two remote Ipsec Tunnel

[nologo73]

Hello,
we are trying to fulfill a complex configuration request from our customer

• They have one PF-1120A in the headquarter (site B). This firewall is connected via IPsec site-to-site to a remote firewall Cisco? (site C)
• Site C is from a different Customer, we can not change configuration nor firewall brand.
• All traffic from site B (many IP client) to site C (via Ipsec) it is NAT to a single IP address. (NAT many-to-one inside the IPsec tunnel)
• They need to contact two RDP server located in site C
  Until here everything works fine.
• Few days ago the customer opened a new branch office (remote site A). In this site he has another firewall brand Zyxel.
• We  successfully created another Ipsec tunnel Site-to-Site between site A (zyxel) and site B (PFsense).

Customer ask if it is possible from all Pc Clients located in remote branch office (site A - Zyxel) contact RDP servers in (site C - Cisco?) passing thought (site B - PFsense) via IPsec tunnel.
We are not sure if this configuration it is possible….... Server RDP in site C respond only to one IP (NAT many-to-one)
We found a doc but we are not sure this is our situation:
https://doc.pfsense.org/index.php/Routing_internet_traffic_through_a_site-to-site_IPsec_tunnel

Hope someone can give us an help
thank you
Luciano

[nologo73]

By chance it is possible to use an OpenVPN tunnel between site A and Site B, and after create a Pfsense rule to send packet from site A to site C?
thank you