Light is light and flash is just another source of light.
Learning to use natural light will teach you some of the basics of working with light, such as the value of diffusion, exclusion and reflection of the light. Those same principles apply to the flash light as well and once you take control over the flash and learn to manipulate and control that light you'll find that is frees you up significantly - in fact you'll find that you've the ability to control scenes and situations with lighting far more than you can now and you'll discover a wealth of options to get closer to the photos you want to take and not be limited only by the available light present in the scene itself.
One of the most basic lessons for flash is diffusion and by that we don't mean putting a little slip of paper infront of it or one of those stofen omnibounces on the cap - those methods (the latter used incorrectly for direct flash) just lower the output rather than actually diffusing the light.
Diffusing it basically involves making the light source itself larger, be that using a softbox so that the light bounces around inside and is enlarged out so that the source relative to the subject, is the big flat front panel of the softbox; or bouncing the light off a reflector or umbrella, again making that second reflected surface into the large light source for the subject; even the omnibounce has its use indoors where the flash head is aimed upward at an angle - in that situation it spreads the light out so that it hits the walls of the building, making the walls themselves into the light source for the subject.
Many people simply fit a flash to the camera, or use the popup, and aim it right at the subject - this produces a very harsh light because the light source is so tiny. Furthermore it always keeps the light only coming one way, right at the subject, this changes the nature of shadows on the subject and often can be very unflattering (unless you are only using the flash with a weak power for fill flash - common in bright daytime - esp with the sun overhead and casting shadows over the subjects face). Get the flash off the camera, gain control over it and learn to diffuse and manipulate it and you'll find a whole new area of photography.