Not sure the difference between 'dropdown' and 'combo' but if I create a combo column with text contents and assign the property validateFilter: false I can free text values into the cell but it will revert the data if I tab out of the cell.
example ref group column: https://plnkr.co/edit/ycXsF49n1OKAqhZC?open=lib%2Fscript.js
Is there a way to customize the combo so that it will accept a free text value not in the list and keep that if I tab out of the cell (i.e. fire it to the updateURL ajaxstore)?