In the current Fusion 360, it is not possible to add/remove some applicable edges to an existing fillet feature. In order to do this, you would have to clear the current selection, and re-select. However, the feature does not retain the previous radius value, and usually the designer does not have this value in the head. You just want to re-select the applicable edges without changing the radius. The current way, thus, is to exit from the feature edit, double click on the existing fillet feature again, remember / copy the existing radius, clear the selection, re-click, and paste the value.
I think if the radius could have been retained it would make the software much more intuitive to use and saves time for the user.
