The icon would still remain 32x32, but you could like, put a border of an area which you couldn't draw one.
So, give it a 1x1 border making it look 30x30, but actually being 32x32.
What would be the point of this?
Why not use DrawBox() to draw those boarders?
In response to Sniper Joe
So we wouldn't draw out of the box without knowing it :p
In response to Nick231
That's at runtime though. :p
In response to Hell Ramen
OH! You mean in DreamMaker! I thought you were making a painting type of thing.

The DreamMaker icon editor is good for pixel editing, If you have a use for the features it would probably be better to get a program that has more advanced graphics editing features (Layers perticularly). There are free ones out there, a search for pixel tutorial will bring up a post with a number of links to free programs.