From Firmware 6.3 onwards, you can map a specific local network / IP address to another, and the packets received by remote gateway will appear to be coming from the mapped network / IP address. To access this feature, navigate to Network > IPsec VPN > your IPsec Profile.
At the resulting popup, click the (?) icon next to the Local Networks menu item.
You can now configure NAT for your IPsec VPN profile. Press the Save button at the bottom, and the Apply Changes button at the top-right corner to implement your changes.