The Sigma 55-200mm Four Thirds lens works with the Olympus EX-25 extension
tube, though the combination allows manual focus only.
Without the EX-25, the minimum focusing distance for the Sigma 55-200mm
is around 110cm (at all focal lengths), giving a minimum field size
(at 200mm) of about 77x58mm - an effective magnfication of about 0.45x.
With the extension tube, the greatest magnification is at 55mm, which
gives almost 1:1 effective magnification at a distance of about 30cm.
A focal length of 200mm at a distance of 85cm gives around 0.8x
So the Sigma telephoto zoom lens plus the extension tube looks like a
decent combination for macrophotography, giving good magnification over
a range of working distances.
One drawback is that depth of field is very limited and getting the
focus right is very difficult (the easiest approach with this combination
seems to be by moving the camera, but that's hard without some kind of
100% crop (with USM)
The EX-25 + Sigma 55-200mm combination isn't as sharp as the Olympus
50mm macro lens, but the two put together cost only two thirds as much
and offer more working distance -- and if you're already carrying the
Sigma lens, the extension tube is more compact and much lighter than
the macro lens.