CamBam can read in stl and dxf files as 3D and 2/2.5D drawings respectively, but not iges or step.
There is quite a lot to learn in any CAM program, so it's going to be more involved than importing a solid and the CAM automatically producing the gcode.
You'll need to use dxf's for 2/2.5D work such as drilling, pocketing and profiling because it's simple and very effective. Use 3D for curved objects like phones, mice (computer and real) and anything else that's not of a mechanical, angular form.
There's plenty to read on the CamBam site too.