It's possible but not easy to do. You will need a retail copy of the Snow Leopard DVD, a 16 GB USB flash drive or similar partition on a USB external drive, access to an older Mac than will boot from the retail Snow Leopard DVD (10.6.3,) and the Mac OS X 10.6.8 Update Combo v1.1 updater.
- Boot the older Mac from the Snow Leopard DVD.
- Partition and format the USB flash drive (or similar partition.)
- Install Snow Leopard on the USB flash drive.
- Boot the older Mac from the USB flash drive.
- Update the Snow Leopard system on the flash drive using the Combo Updater. (Alternatively, you can install all the updates that appear in Software Update.)
- Boot your new iMac with the USB flash drive system.
- Create a new partition on the new iMac's hard drive.
- Clone the USB flash drive to the new partition on your new iMac.
If all is well your new iMac will boot into Snow Leopard. Note that you cannot use Boot Camp Assistant to install Windows if you do this.