If you need both way access, you need to add your subnet to the local subnets in phase2 just like customer's. But if you need just one way from your subnet to the customer's remote subnet, you need to grab one customer's local IP and put it into IP Pool and set SNAT in a policy to pretend all of your devices are one of customer's local device. You might want to avoid IP conflict by somehow reserving the NAT IP not to be used by the customer.
Or use the combination; assign one IP the customer would never use and put that /32 IP in phase2, and then NAT your access to the remote side with that source IP. It's still one way though.
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