A powder is de-aerated in a collector and then transferred to a chamber via a valve. The chamber may then be injected with a low-pressure air stream carrying an additive for mixing with the de-aerated powder. The mixing of the powder with the air and additive stream may occur in the valve and/or chamber. After mixing of the powder with the air and additive stream, the mixed powder and additive may be further mixed in the chamber and/or second valve prior to transferring the mixed powder and additive to a holding container or other processing device.