Hi Gordon, yes some wood burning stoves can be converted, but most can't.. I have two wood stove fireplace inserts in my house, I'd love to burn a bucket of coal ocassionally in them.. but there is no way to get all combustion air to be routed to the underside of the fire... there are no grates, and If I installed some grates, ther would be no ashpan, or air control to the fire... they just won't convert to coal.
For a good deal on a coal stove, keep an eye on http://papershop.com/ http://www.craigslist.com
and your local papers... ask neighbors about anyone burning coal.. you may find a stove in your neighborhood. Don't forget http://www.ebay.com
. many good stoves come up for sale there too.
The Hitzer stoves, the Harman stoves. the Alaska stoves, and several others are very good hand feed stoves.
After looking at the cut-away drawings of the stoves, you can see that there are several required items in the design:
combustion air vents usually through the ashpan area to feed air under the coal bed
Shaker grates, hopefully with an external handle
Usually a firebrick lined firebox
Often a heat distribution fan, which can be used or not, depending on desired heat output
Hope this helps... Greg L