How to make To Do list tasks calculate cost

We use To Do list tasks to capture work items that fall outside of our current projects, some of these might also be billable (as they are work for a customer). However, we really don't want to have to set up a project for all these random tasks (especially as we use Team member licenses).

I currently have 2 custom fields (hourly rate & item cost) that get updated by a workflow when a timesheet is submitted/approved on a to do list task. 

This works, but feels like a workaround as all the fields would already exist, but seem to be disabled for TO DO list tasks.