Using an existing template is easy, when you create or modify a Data Generator Project, and the selected column has a data type that is supported for Templates (see Introduction to Templates), there will be an entry Template in the Generate column/parameter settings.
Once you have selected the option to use a template, the Template drop down box will list the available templates for the column data type. If you select a template from the list, the column will be "ready to fill".
If you save the Test Data Generator project, the project <-> template link is stored as a dependency, see Deleting a Template for information template-dependencies and how to recreate links.