Site A to B to third party C requiring NAT

[trueinnovation]

Hi,

I have an existing, functioning site to site IPSec tunnel from our site B to a third party Site C.
Our internal subnet:
10.177.140.0/24
We NAT this to a public IP:
100.0.0.1
Their network (single public-range IP):
200.0.0.1/32

So, all traffic from our private subnet on 10.177.140.0/24 goes to their public (but only accessible via VPN) IP of 200.0.0.1 and presents as 100.0.0.1 to them. This works and is stable.

We now have a third site, site A, which is controlled by us. We need it to talk to Site C, but without building a tunnel to it, as they will only support one tunnel per customer. We also must still present this traffic as coming from 100.0.0.1
Our internal IP range there at site A: 10.177.150.0/24

How can we accomplish this in PFSense, if it's possible at all?

Thanks for any input!