Assuming your 620 is the same as my 695 (and I'm reasonably sure it is), you just need an adjustable link, like what I used to raise my 695.
The standard link on a 695 is 275mm long. I bought a link from a 748, which when I pulled it out of the box was ~270mm long, which would have dropped the back by about 15mm. I could have made it shorter, but only by maybe 5mm more, for a total drop of ~30mm.
Here's one on ebay (from the same seller I bought mine from):
http://cgi.ebay.com/ebaymotors/DUCATI-SWING-ARM-RIDE-HEIGHT-ADJUSTER-ROD-748-998_W0QQcmdZViewItemQQhashZitem1c118c396bQQitemZ120553486699QQptZMotorcyclesQ5fPartsQ5fAccessoriesHere's the specific post about what I did in raising my 695:
http://ducatimonsterforum.org/index.php?topic=36816.msg653287#msg653287If the 748 linkage doesn't lower it enough, then you can just make a shorter threaded rod, though one of the threads is annoyingly left-hand.
You can also buy aftermarket shorter rods.