I see that there is a Priority field within the process (workflows) data structure. You can enable it by editing a report such as 'Processes requested by me' and moving Priority from the Available to the Selected column. However I cannot find a way to make this a field in my workflow forms. If I create a custom field named Priority, both are listed (with the same name) in the Report customizer
Also, to avoid confusion, Celoxis should consider not allowing custom fields with the same name as internal columns.