Perhaps you can try setPos'ing a non-exploding vehicle (such as a jeep) under the spot from where you want the smoke to emit. Then, using a script, periodically set its damage to 1, then back to 0 when the smoke is about to stop, then back to 1, and so on.
Otherwise, for constant gray smoke, you can setPos a camp fire below the ground and have it burn as long as you need smoke.