AddColumns does not add a column to a table or collection. It only works with tables.
Currently we have to use a ForAll loop example:
Collect( 'Sandbox Building', AddColumns( OpenIncidents, "Distance", (IncidentLat-Lat)+(IncidentLon-Lon) ) )
I'm recommending you allow me to just put AddColumns(Sandbox Building, "ColumnInCollection", (Column1 + Column2);
it'd be nice to see this as collections are also just temporary.