No, the software will create extra frames to fill up the space needed - these frames are basically two pictures combined in a "ghost"-like way:
Where normally at 12FPS you would have frame 1 on the left and frame 2 on the right, now at 24FPS you would have frame 1, then 2, then 3. The one in the middle is a combination of the other two combined. Of course, this isn't "motion blur", technically.
Proper motion blur is more complicated: I myself tried it out using GIMP for a bit. I'm sure there is software out there that can do it automatically, but I don't know of any free ones.