@ethan-103
You can do this with BINAT for sure, but this requires a policy-based tunnel.
With VTI you can configure a NAT 1:1 to achieve this.
For example 10.0.20.0/24 would nat to site A 172.16.5.0/24 ( 10.0.20.100 = 172.16.5.100)
For this example you have to add a NAT 1:1 rule to the VTI interface at A, where the "External subnet IP" is 10.0.20.0 and the "Internal IP" is type Network > 172.16.5.0/24.