Don't know if it's the "right" way, but you can choose Bottom (or Top) Mask from the drawing options, then select the line drawing, and draw lines of appropriate width (set width in the properties dialog on right side of the window) where you want the mask to be excluded, this works (in the 3d preview at least).
I did an example for you, first image shows settings and line drawn (note that line appears under the trace, weird, probably a bug maybe) and second image shows the 3d preview, the dark green is the board, the lighter green is the mask, so you can see that the mask has been excluded where the line is.