Many to one with static NAT

I have a range of public IP addresses, let's say 1.2.3.0/24. Each IP is NATed to the same IP address in my DMZ, which is a "load" balancer (called LB here): depending on the URL requested, it redirects the traffic to the good web server (with the good virtual host, so it's more a redirector than a load balancer).

Each time I host a new website, I have to :
1) Create a host object with the IP address 1.2.3.x
2) Create a NAT rule nating 1.2.3.x to LB IP address
3) Then another admin configure the LB, that's ok, not my stuff
4) Allow the traffic for HTTP/HTTPS

That means for N websites, I have N objects, and N NAT rules.

I would like to NAT my 1.2.3.0/24 range to the LB IP address in one rule (it's not a N to N NAT but in this case a N to 1 NAT), so I don't have to create an object and a new NAT rule each time. All the range is NATed an reaches the Load Balancer (and then it answers or not).

* We don't want to manage a 10.0.0.0/24 range on the Load Balancer to do a N to N nat, to much work load.
* I need to have a public IP address per website (it's not possible in my situation to host everything with the same public IP address).

I tried to do it but it looks like it's not possible. How would you do this ? (NGX R65)