Did you read the first post in this thread?
Technically, you can add it, but it is not an officially supported method in part because it can wreak havoc with your zigbee network. The hue bulbs do not do a good job of repeating messages for other SmartThings hub-connected zigbee devices like battery-operated sensors, so messages from the sensors can get lost, making them unreliable.
Also, when connected via a hue bridge the bulbs can operate locally. That is not true at present if you are using a custom DTH.
So your entire network will run more smoothly and the bulbs will probably be more responsive if you connect via a hue bridge. The other way is technically possible, but not recommended.
If you want to use bulbs without a bridge, get the Ikea Tradfri. Excellent zigbee repeaters when used with SmartThings, and very inexpensive. 