If you dont want to change IP addresses, you can try configuring port-based VLANS. Assign a VLAN on the port that connects to your device. Assign an interface to that VLAN and give it an IP address of the same network. VLAN information: Here, here and here.
However, when your device receives traffic from other networks, it does not know where to send the packets. Usually, the devices use a default gateway, but in your case this option is unavailable. The only address your device is aware of, is the address of the interface it is directly connected to. The SNAT on RUT950 would allow you change the source address of the packets to one of the RUT950 interface's (The IP address of the interface directly connected to your device). You can find more information about SNAT here.
The other end of the IPSec tunnel also needs to be aware of the new VLAN network, so you will need to add that network to your IPSec configuration as well. Unless IPSec tunnel is a default route.
I have not tested this scenario myself. Please, if you have any problems, let me know more about your case and I will try to test it. A more detailed topology and a troubleshoot file would be great.