I've done 2 batches of lagers that got bottled. Neither required extra yeast. I've been told that there is usually (barring mitigating factors) plenty of yeast in the beer even after fermentation is done. In fact the only time I ever had problems with bottle carbonating was with an ale that was in a slightly too cold room, and there all I had to do was turn the heat up a bit in that room and eventually they carbonated (the yeast was there all along, just too cold to work)
Another issue is at what temp to let the bottles age while carbonating. I've done that at room temperature, the logic being that the amount of sugar you add is so small relative to the original malt that any off flavors from a "too-high" temperature are not noticable, and this way you can just let the bottles age in some spare room of your house, rather than having to take up space wherever you did the original fermenting. But you can go either way, room temp or ferment temp- the one thing that probably won't work is to age the bottles at lager temp.