You should just be able to use the "drag and drop" behaviour that comes with Stencyl, combined with some logic that gets the actor's x/y positions and rounds them to the nearest grid line. That would look something like this:
set x : round([x of self]/[size of grid width])x[size of grid width]
Repeat for y.
The problem you'll face though is that it will snap to this grid all the time. So you're going to have to look at making it only snap if the actor is within a certain limit of the grid. I might do it for a future pre-shipped behaviour but for now you will probably have to do this for yourself, although I can probably help.
