What I've found that although Studio doesn't see a PShop's transparency from a layer mask when sending that layer to Studio, it does leave it alone. So, if you make adjustments in Studio & OK to send that layer back to PShop, the layer mask remains intact. While not quite the same in that you don't get the interaction between layers below as you would in Studio, it's a start.
What would be nice is if you could somehow at least copy & paste or import an external file into Studio as a mask.
The odd thing is that some of the PNG's used with Texture effects have embedded transparency that Studio DOES respect. Haven't figured out what's different in order to make my own though.