Of course, but the two should be at least an architectural match, or they should be complementary. The foundation is most important..And then there is the extra cost of precisely locating the addition...
Whether or not it is worth it is a different matter.

Yes, in the fall 2005 issue of BH&G Simply Perfect Additions there is an article about where someone did just that! I will be happy to mail you the article if you can't find it. Just let me know.
Mary

I've worked on designs for a number of modular additions, including many where the roofs have been removed from single story houses and full second floor additions have been set. Many modular manufacturers do this on a somewhat limited basis, sometimes through builders who specialize in this process.

I have talked to several modular home builders about a modular addition to my existing (stick built) Cape Cod. There is universal lack of interest owing to the size of such a project being too small to make money on. Maybe a large addition to a very large house will excite their interest....

I've worked with one particular builder on Long Island, NY whose business specializes in just such modular additions to small Cape Cod style houses. In that locale, building lots are growing scarce and often the only way to expand a house is to go upwards.

There just may not be enough interest in your area for a builder to invest time in perfecting the process.