With OpenSCAD, you don't have fillets as part of the language, but you can do chamfers or machinable parts easily using the minkowski sum operation, which basically places an object of your choice on every point along the surface of another object. I've used this to do insets and outsets, radiused corners, and all sorts of other funky stuff.

However, if you want that sort of operations you would probably be better off with cadquery, which is basically the engine behind FreeCAD, with a really powerful Python interface. It has native support for face operations like fillets.