I'm pretty sure there was a single shot version made in China (that's memory talkin' and it's quite unreliable) for training. I haven't seen one converted, but that might be that the cost of converting it would be greater than the cost of an average Mosin. Convert the bolt to work with rimless, remove/shorten barrel and recut chamber.
There are chamber adapters to convert different calibers to use 7.62x39, but I have only found them to convert .308 (7.62x51) in Canada. (Imported from Alaska)
With deep enough pockets it could be done no doubt.
EDIT:
Found a reference to the single shot I mentioned, and a link to another fellow with similar intent:
http://www.homegunsmith.com/archive/T16487.html
http://www.russian-mosin-nagant.com/global/index.html
In looking for the above found a wiki reference to an M/56: An experimental 7.62x39 version in Finland.