Simply create new dummy Gateway object without defining its topology using one of the loopback IPs, for instance:

Change your "Install On" in NAT Properties of the objects slated for Static NAT to the dummy gateway:

Define policy installation target as "Specific" and point it to the gateway it is originally designed for:

Publish changes and install the policy.
Subsequent NATs from these hosts will be subjected to the NAT applied on the Network object, if any:

Of course, you can script the object's NAT target change to do this in bulk, once the dummy gateway object is created.