Hey guys, sorry if this has already been asked, but i'm new to Dishonored and was wondering, when i download The Knife of Dunwall, can i play it in my current save? or will i have to start a new game to play it? thanks.

The story is standalone, so you don't have to worry about it overwriting your main save files.

Yep, what inDIGnation said - the DLC provides some more missions which are completely separate from the main game, so you don't have to start the main storyline again.