Modifying the G code

The hardware is capable, but there are a lot of pieces involved in getting arcs to work. Marlin supports Arcs (G2 and G3) but to get Arcs to work it starts with your model. If you are using a 3D CAD application and exporting to STL you already lost the battle, since the conversion to STL will make all you curves faceted. So you need to find one that can export in a format that a slicer can understand. Then your slicer has to know what an Arc is and output it correctly in g-code. That just won't happen all by itself.

Your second alternative would be to program the g-code yourself for the entire model, by hand. You will not be getting any help on that one from anyone on this forum. There might be some CNC people that know enough about g-code that they might be able to help. Again, likely won't happen.

Your third alternative is to use the tools you already have to their best advantage. While not using Arcs you can minimize the faceting by making sure you export your STL in high resolution and not with a limited amount of triangles. If you use openSCAD or some other mathematical CAD program, make sure your curved sections are designed at resolutions greater than 1° per arc segment. Then when you print it, even though it may still be faceted, the facets will be small enough to barely be noticeable.

Like I said in the beginning, the R1 can do it if given the correct commands, it is just getting the correct commands to it in the printer sphere is not as easy as selecting a checkbox in some CAD program or slicer dialog.

Special Note: Don't even think about converting an already sliced model by trying to replace all the moves for your arc with a G2 or G3.