I just purchased Canon 6D (Full frame camera) and I hate to lose the usage of my simple 18-200 EF-S lens. Any idea if there is a technology that allows me to use the lens on the 5d body?
Is it possible? Yes. You can probably do this by physically modifying the lens.
Is it safe? No. The rear element of some (most?) Canon EF-S lenses extends into the body when at the wide (18mm) end. This causes the mirror to hit the back of the lens, potentially damaging both the lens and the camera.
People have had success modifying EF-S lenses to fit on APS-H (NOT full-frame) bodies such as the 1D series, though the lens may not be safe to use at all focal lengths.
PS: You might be able to use it if you find an appropriate extension tube, but you'll lose infinity focus. As far as I know, modifying the lens or using an extension tube are the only two ways.
I know it was already mentioned, but I'd chiefly say use an extension tube. It both gets the optics away from the mirror to protect it and it also gives the image circle enough room to expand and cover the sensor.
Think about it this way, normally an EF-S lens is only concerned with throwing an image circle that will cover the APS-C lens, but when it is pointed at an EF sensor, it doesn't cover the whole sensor so parts of the image would be black. Adding an extension tube lets the circle get bigger (and thus "multiplies" the focal length). This is the same as the principal that makes it so that the "effective focal length" of a lens is longer on an APS-C body since the sensor only occupies a fraction of the image circle produced by an EF lens.
There is no current available technology to do that and you probably wouldn't want to because the image would not cover the whole 6D frame unless additional optics were involved.
Extension tubes (which are available) would result in restricted focus range. You wouldn't be able to focus to infinity. They would be usable for close focus work if you were only really interested in the central region of the frame.
If there was a 1.4x or 2X extender that would allow EF-S lenses to be mounted on it as well as regular EF lenses and which itself was EF and not EF-S, then your problem would be solved (at least mechanically and electronically). Image quality (not great to start with) would suffer though. The Canon extenders will not allow that and my Tamron extenders will not allow it either. If any of the 3rd party extenders allow EF-S lenses to be mounted, I'm unaware of it.
Way back in the early days of EF-S lenses I did physically modify an EF-S 18-55 to mount on my then EOS 10D body (Crop sensor but not compatible with EF-S lenses). Here's a link to that article: EF-S lens conversion. Note however that the larger mirror of a full frame body could (and possibly would) hit the rear element of any EF-S lens modified this way and mounted on a full frame body.