A extended magazine would add more weight to an already heavy gun, that is why the russians switched to the 7.62x39. With the new round they were able to reduce weight and allow more ammo to be carried, the 7.62x54r round can take up quite a bit of space. Besides, the 7.62x54r round is powerful enough that whatever you hit, it should damage and/or destroy.
Never seen one, and there probably never will be as the magazine is rather complicated compared to most. It has a interrupter to keep you from picking up 2 cartridges because of the rimmed case design and it forms the trigger guard as well.
There are some or were some 20 round magazines developed for mauser rifles patterned after the trench magazine used by the Germans in the first world war. If you want a high cap bolt action rifle, Get a Mauser with the afore mentioned mag or Lee-Enfield with its standard 10 round mag.