The first and easiest thing for this is to get a 52mm close up filter. Example of this would be the Hoya 52mm close up kit I'm going to start out with pointing out that this will be less than ideal in terms of quality (you're putting a single element uncorrected lens in front of your lens) and possible mechanical vignetting (where the filter obscures part of the image) - similar to the "you have a lens hood of the wrong size" problem.

For extension tubes, the Kenko set will work.

This DG Extension tube set has upgraded "Gate-Array IC" circuitry to work better with digital SLR's and some of the digital SLR lenses, like the Nikon DX (e.g. 17-55mm, 10.5mm) digital-only lenses.

The thinest filter in that set is a 12mm thickness.

The native magnification of the 18-55 claims a 0.31x magnification natively, though I'm not sure which end of the zoom that is on. If its on the short end, this would get you to 0.98x magnification. If its something less than that (like 0.15x native), this would amount to 0.82x, which is still more magnification than your minimum acceptance criteria.

I will point out that with the extension tube you will be losing quite a bit of light. The effective aperture at 18mm will be f/(3.5 * (1.9ish)) or about f/6.6. You will likely have trouble with autofocus at that point getting sufficient light in there to work satisfactorily. I'm also going to point out that your depth of field for macro at wide angle is going to be paper thin... one calculator I plugged the numbers in with a .9x magnification ratio starting from f/4 gave a depth of field of about 0.35mm.

I don't think that the closest focusing will be behind the front element. From my days back when I was playing with a 17-35mm f/3.5 sigma zoom lens on film and extension tubes I remember photographing a flower... and once I got it in focus I went to look at where the flower was in relation to the camera... and it was a small fraction of an inch from touching the front element. That was a much larger (and physically longer) lens than the 18-55 (getting an 82mm close up filter was kind of out of the question (and thats the cheap brand)).

I don't think that you will want to be photographing at 18mm for macro given the other limitations that you will encounter, but then again the only way to figure this out is to actually do it.

---

Going back to your these photos, they are not shot with either of the setups that I describe. There is far too much depth of field (remember that I was talking about 1/3 of a millimeter depth of field) and there are some wildly uncorrected bowing of the horizon (barrel distortion) when it isn't across the center of the lens (for example this and this) which leads me to suspect it's a fisheye lens just shooting really close (the 10.5mm fisheye it has a native magnification ratio of 0.20x and closest focusing of about 6"). The 0.20x would be sufficient for the images shown - you don't need to go to 0.5x to get those images.