Yes. If you create a custom part. The copper layer is an svg.
There are a couple of ways. Most flexible is to make a second copy of the sketch which creates the solder mask layer you want, then substitute the solder mask layer from that gerber output with the solder mask layer of the actual board you want. The second method “invisible copper” (a copper layer with fill=none) fools the mask creation code to not put solder mask where there is “invisible copper”. That method is described in this forum post.
Peter