There are two ways to do this...
This is the "easy" way. Since HD is (by definition) a frame size of 1920x1080, using a bigger set of images than 1920x1080 means that you can pan around in that frame using your post-production software of choice. For example, I'll often take timelapses using Medium sized JPEGs (4080x2720 on my Canon 5DmkII) and that gives me all kinds of room to add movement in any direction (or multiple directions) as I see fit. The downside to this is that it isn't "real" motion, so to some extent it looks... well... fake. This is because with a real pan the foreground objects and the background objects would move at different rates (this is called parallax), and in this solution you're simply sliding a video with no motion around a frame. Many (if not most) post production video editing suites will be able to accomplish this without a problem... I do it in both Adobe Premiere and Adobe After Effects right out of the box.
This is the "hard" and "expensive" way. Simply put, you move the camera. There are lots of DIY solutions for accomplishing this, but (as of this writing) there is no out-of-the-box and off-the-shelf solution available. As the topic of building your own motion rig is a bit (OK, a lot) outside the scope of photo.se.com, I'll speak in general terms and say that you'll be using a combination of microcontroller-based stepper-motor control (such as Arduino, or those produced by Parallax, Inc.), sliders/dollys (such as the Pocket Dolly by Kessler Crane), and/or pan/tilt heads in order to achieve motion. I've put more than $2,000 and hundreds of hours into the construction and programming of my motion control rig, so it's not an undertaking for the faint of heart. :-)
The benefit of this method is that it is true motion. Parallax is present, and it looks great. The downside (obviously) is that you need to have a fair amount of solid-state programming knowledge, be handy with a soldering iron, and have the money to put it all together. If you're interested in exploring this option further, there are lots of us over at the timescapes forums that are developing lots of different approaches to this method of motion control.