Yes, you can combine an external flash with a ring flash, but whether it’s worthwhile depends on your macro shooting style.

A ring flash is most useful when you want a very mobile, handheld setup and need even light very close to the subject without the lens blocking the flash. That’s why ring flashes are common in macro work.

If you don’t need that mobility, or you’re not shooting handheld, a ring flash usually doesn’t offer a major advantage over an off-camera SB-900—unless you specifically want the distinctive flat, even “ring flash” look.

For many macro setups, one off-camera flash plus reflectors can already produce excellent lighting. A second regular flash can also give you more flexible directional lighting than a ring flash.

So:

• choose a ring flash for close, mobile, shadow-free macro work
• choose one or two off-camera flashes for more flexible lighting control
• don’t overlook reflectors with your existing flash