Private IP adressing

Customer has an old production plant where a PLC resist with an IP address 100.100.100.6/16

They asked me to NAT this internal IP address via a FW to an address in a 10.28.198.0/24 network.

First question: is this possible?
I'm not a networking expert but 100.100.100.0/16 isn't part of a private IP adressing range. The FW is a Siemens Scalance S615, when I try to enter the 100.100.100.6 IP adress in the NAT rules, I recieve the warning that this is an invalid IP address.

Should I advice the customer to first change the IP adress to a usable private range?