Add existing custom fields to team-managed projects e.g. Tempo fields

As a user working with a Next-gen project I would like the ability to add existing custom fields to the a project. 

My company uses Tempo for our time tracking and we have invested some time in integrating tempo accounts with our CRM to keep an up to date list of possible client's to track time against. We would like to use the new next-gen projects but we are unable to add existing custom fields so although the tempo plug in works to log work we can't associate that work with a specific client.

This would also be useful for other fields which are common to many of our development teams. For instance we have dropdowns for other business units that need to review our work that should be consistent across projects. Maintaining these separately in the current next-gen projects is onerous.