Sometimes you may want to automatically update a field through a workflow, but don't want this field to be modified by the user. If a field is read-only you will find it is not possible to assign values to it on a workflow. To work around it, follow these steps:
1-Uncheck the "field is read only" option on Settings>Customization>(Entity)>Attributes
2-Access the workflow and add the properties you need on the field.
3- Go back and set the field to read-only once again.