Unfortunately the table PM_ProjectTask is bound to the class PM.ProjectTask of the "Project management" module and that class is not editable because the module is installed.
How can I edit it?
Anyway, I tried the following example about the customization of system's data classes:
https://docs.kentico.com/pages/viewpage.action?pageId=7471613
but still I don't know how to add the new column in the databse by Interface or Kentico API and write values in that column.