How do I add more fields to the Service Desk Request form

[Carlos]

When I am in Portfolio Roadmap view.  There is a dropdown fro "Filters".  It has only a set of fields and it does NOT allow to add any other field.  For example in that view, I would like to filter by "Assignee" which is not one of the preset fields. 

I know I can use other Jira interfaces to get that info, but we would like to have that option in the Portfolio view.  Any help will be greatly appreciated.

Thank you.