In order to add a custom task first navigate to the job that you want to add a custom task to. Then click the action button in the bottom right. Scroll down the list and you should find the option "Add Custom Task".
This will take you to this screen where you can setup the newly created task. From this screen you can select preceding tasks and dependent task, as well as other details for the task.
When you are happy with your changes press "Save Custom Task" at the bottom'.
To do this you need ClickHome version 3.4 or above, user needs opcode 7123 to allow access to screen