Extension objects are just custom data visualizers, they do not aim to modify the document data. You can have a look at the DynamicDataUpdate function of the automation API to fulfill your need, AFAIK it is the only way to change already loaded data of a QlikView document. However I don't know how you can call this function from an extension.
I know QlikView is not intended for altering data, but I want to explore this area, especially now with the extension objects.
Well, I don't want to change the actual source data, but I want to save the changes and then in QlikView have the original values be replaced with the changed values. Sort of like how the QV inputfields work right now, but they don't have validation and such.
The only connection back to QlikView I've made is the selection of rows with _this.Data.SelectRow(rowID) which is based on the first dimension and also picking more specific selections in other dimensions.
I can change the value of _this.Data.Rows[row][column].data but the changed value is only stored locally in the Extension Object and doesn't change in QV.
Hope this makes sense