How big is the plug you're cutting off? Is a wall wart sorta looking thing?
Also, how many LEDs are in the string? Are we talking a dozen maybe or more like 100?
My first thought is that no, you would not be able to do this. Yes, LEDs are DC devices, making DC from AC is one of their primary uses. What is likely is that the string won't light.
Good news is depending on how the string is made you might be able to re-wire it to work like you want.
"Stand your ground. Don't fire unless fired upon, but if they mean to have a war, let it begin here." -- Capt. John Parker