Feature request from mage2pro, posted on GitHub Jan 27, 2016
An example of the UI component XML declaration:
Comment from tkacheva, posted on GitHub Jun 30, 2016
@vkorotun please take a look
Comment from tkacheva, posted on GitHub Aug 19, 2016
internal ticket MAGETWO-57229
Yes, UI Components seem to be incomplete, this functionality is important, Ui Components allow you to easily create grids and forms in backend, but it is difficult to add fields to existing UI Components.