How to add transparent padding of fixed number of pixels to an element?
I do not want initial image to change size, I simply want to add transparent padding around what I already have.
Draw a rectangle the size you want. Set it to have no fill and no stroke. Align your art to that rectangle. (it may be easier to give it a stroke until things are aligned, then remove the stroke to save/export)
From there, you can either convert it to an artboard (Object > Artboards > Convert to Artboard) if you are using CS4+ or you can just leave the rectangle and exporting/Save for Web will see that as the artwork boundaries.