How did you add/edit the modbus data things for your configuration? You must do it the same way. It’s just another parameter for a data thing.
Guessing you are using PaperUI, find the ‘read transform’ box and put in it
JS(divide10.js)
The JS parts tells openHAB it’s javascript, because there are several types of transformations
The divide10.js is the name of your file in /conf/scripts/