Custom Global Merge fields do not require a Doorways Supported merge field. Users with given permission have the ability to create a custom merge field set by the user themselves:
Guide to Creating Custom Merge Fields:
A. New Merge Field Button: Clicking this button starts the process of creating a new merge field.
B. Custom Merge Field Info: each row contains data pertaining to an individual merge field. This information includes the Name, Description, Type (image or string), and Value.
C. Edit Button: Click this button to edit the name and/or value of the merge field.
D. Delete: Click this button to remove the custom merge field completely.