Security like that is not built in to Unitronics, as you've concluded.
One thing I've done is to roll my own authentication code based on a collection of mathematical operations. Each Unitronics PLC with Ethernet has a unique MAC address located in SDW 22 and 23. You could keep track of these in your server an create a code to send along with your data. Put logic in that matches the reverse of your math operations and use the result to verfiy the proper data origin.
You have not given the details of communication. If you are going through the Internet, there are already many security methods available to clamp down on unauthorized data transmissions. This may be a better question for a good IT person.