I have been exploring how to do this. I would think the best option would be to store in a related table that allows one to open in collector and add related records. Has anyone created a table and established a database relationship to the assignments table in Workforce? If ESRI is listening, it would be really good if the Workforce apps supported editing related tables.
Unfortunately, neither of these requests (Tracking Costs and Activities Associated with Workforce Work Orders) are on the roadmap for Workforce. We've decided that these are outside of our current product plan for Workforce. We have several Business Partners who offer solutions that do tailor to solving these requirements.
With respect to adding the ability for Workforce to edit related tables, this is also not on our roadmap. We don't have any plans to extend the assignments feature layer to support related tables. If we do consider this to satisfy a use case, then that would provide the opportunity to add that ability.
Is it possible to create a table and relate it to the assignments layer and then use collector or a WebApp Builder app to edit. That way the field user could at least enter notes about time and materials so the back office can enter and correctly code them to our general ledger accounts.
Yes, that should be possible.
One thing to note, we do have it on our roadmap to support editing and display of non-workforce fields in the Assignments feature layer. Not quite a related record approach, but that could meet your needs when that is implemented.