Selecting data columns from both Contact custom fields and adding data to it is available in all 
Global Data sections but, creating new Contact custom field is available only in Contacts section (Learn more here).
Here's a step-by-step guide on how to add data to already created Contact Custom field:
Navigate to
Global Data → select section where you would like to add data
Double check if the Custom field column has been added already to the section view
if not, click on the Columns, search out contact field and include it in the view
Locate Custom field column inside the section view
Add data by clicking on a "Pencil" button inside the field for the specific contact
Adding data options available will appear depending of the custom field type
(e.g. if type is short or long text, you will be able to enter text, but if the field type is to select one of your Users, the drop down menu with available users will appear)