Campden will not kill your yeast. If you add any sugar back to your wine, it can and will start fermenting again. You can stabilize your wine with Sorbate (and campden) to prevent renewed fermentation if your wine is pretty clear already (low yeast population). There are other alternatives -- you can add some commercial (or homemade) wine that is similar in character or style to top up. You can rack from larger demijohns to smaller ones. I generally keep a collection of all sizes available for this from 6.5 gallons down to 1 liter. The other thing you can do is add clean marbles to your demijohn to make up for the lost space. Or you can simply top up with water. If you make a stronger higher alcohol wine to begin with, the dilution will not be that much of a problem.
--Greg