You won't be able to unmask specific traces but there is a way to get the same effect. Remember that solder mask layers are negative: anywhere you have a solid shape, you will have a "hole" in the mask. First make the appropriate solder mask layer (top or bottom) the active layer. Select the drafting toolbar. Create a Copper polygon wherever you want an opening in the solder mask. Verify in the CAM viewer or a third party Gerber viewer.
I have done exaclty what you say. I've got the solder mask I want.