Hey @Dave_Burns, welcome to the forum 
Just to ensure I understand correctly you are trying to have custom fields added to tasks that are private and not in any project?
If so then the fastest way is to add the task to a new project where you add the custom fields then you can update those.
You can also add a task to more projects at once then all these custom fields would show.
More info How to Create and Use Custom Fields in Asana | Product Guide • Asana Product Guide
Did I understood correctly what you are trying to achieve? Does the above help?
P.S. you might also want to vote for Custom Fields in My Tasks