Form based on a query in the datasource cannot show new tables added to the query. All though the added datasource is visible in the query on the form it is not included in the list of available datasources from which you can drag’n’drop fields into the grid.
To make that happen you need to do a little massaging of the form outside AX.
Export the form and edit the export file in Notepad. Locate the definition of the datasources and add the following text (remember to change the name and table to whatever fits your current situation).
The text should be inserted immediately after the other data source objects and before the derived data source objects:
DATASOURCE OBJECTPOOL PROPERTIES Name #<DatasourceName> Table #<TableName> AllowEdit #No AllowCreate #No AllowDelete #No OnlyFetchActive #Yes ENDPROPERTIES FIELDLIST ENDFIELDLIST ENDOBJECTPOOL METHODS ENDMETHODS LINKTYPES DATASOURCE PROPERTIES Name #<DataSourcename> ParentDataSource #<ParentDatasourceName> ENDPROPERTIES ENDLINKTYPES ENDDATASOURCE