SNat on Portmap - how do I do this?

[tverweij]

I have the following situation:

An Ip (from the internet) connects to the firewall.
IP -> WanIP

This IP is in a specific alias, and therefor a NAT rule is executed, changing it in:
IP -> DestIp

DestIp is located on another server, what means that the traffic is routed through an IPSec tunnel
IP -> IPSecAdapter -> Dest IP

On the other server, I see the traffic coming in:
IPSecAdapter -> DestIP

So far, so good.
But now the way back.

For the destination server, the connection comes from IP, and IP is on the internet.
This means that the routing goes to the WAn op the second server and all replies to IP get lost into the void.

To solve this, I need an Outbound NAT on the same traffic as the PortForward is executed on, changing the traffic flow in:

NATIP -> IPSecAdapter -> Dest IP

But whatever I do, the outbound rule won't execute on the traffic.
I can execute it on the IPSec adapter, but when I do that, no traffic reaches the other server anymore.

How can I solve this?

[tverweij]

To make the question somewhat simpler:

How do I apply a source-nat on a portmapping?