how to modify data to be presented by Editor

use case: datatable displays rows of Orders. Each order has a property called "hasRMA" (return merchandise authorization), and each table row has a button - either "CreateRMA" or "EditRMA", which is bound to this property. Editor is to be used to Create or Edit RMA. However there is no RMA object in the Order object; instead for Create - an RMA object is to be (new'ed) and attached to the Order as a new RMA property. I was hoping to do this in the "preOpen," by finding tbl.row(editor.modifier()).data() which does get me to the Order object for this row. But adding a new RMA property doesn't do anything (doesn't show up in the Editor).

SO: how can I modify the object that the Editor is about to present?