I actually came across this thread Custom shapes for the solder mask blocker only a few days ago from more than a year ago where someone else asked the same question. In it some one provides an example of how to do just what you are asking. I downloaded the example and it looks like you make a part just like you would for any other part except you do not use a fill or stroke on the svg element defining the copper. This means it does not get rendered or used during copper export but it does get used when the solder mask is calculated. It will also crop any and all silkscreens from the same area.
Now with all that said making your own parts can be frustrating and takes awhile to get comfortable with. This is not helped in that you can make parts in multiple ways and the instructions around can seem conflicting. If you do not have experience and you would like to learn just let us know and also let us know if you would like to learn by manipulating XML data or by using a the GUI.