Hi Laurence,
I haven't heard of anyone doing this. Usually people want to split sites into separate databases, not merge them into one. Splitting sites into new databases is difficult enough, I am confident that merging them is even more difficult.
May I ask why you want to do this? My company hosts a lot of mojoPortal sites so I may have another solution for you if you can tell me what your ultimate goal is.
Thanks,
Joe D.