If they are still images, you can do this in most any 2d image-editing app that supports 32 bit images. (RGB+A)
If you've rendered out moving footage, then I'm not sure about free compositing software
I think Mike Mckinley knows of a free dv package that might support it.
I'll see what I can find, and repost anything I find
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com