Solved
Workfront API to retrieve User Custom Fields
I thought this would work similar to retrieving Custom Fields in a Project, but its still only pulling in the basic fields.
GET
https:// <MyDomain>.my.workfront.com/attask/api/v15.0/user/5edb22c5022aac5751a37c4d0acfc53f/fields=parameterValues
Any help is greatly appreciated.