Adding offset to mask

Hi I'm trying to make masks for my game sprites with the aid of the cool animation features of RWP.
Once I have a strip like this:
http://s5.postimg.org/tc6hqojzn/jetpaco_izda.png
in order to get the masks of every frame, I press 'Invert Selection' and fill them in black with the bucket tool. Here you can see the masks:
http://s5.postimg.org/tqv96d3xv/jetpaco_izda2.png
What I'd like to know is how could I add an outer offset of one pixel around the selection before filling it in black. Thanks