You can't import cities into Toy Town, but you can import them into City Viewer.
How do you import a city from the other Medieval City generator?
Click "Export" in MFCG, then choose "JSON" from the menu. The city will be saved as a JSON-file on your computer. You need to drag and drop that file onto the City Viewer. Yeah, I know it's neither convenient, nor intuitive, but it's versatile and this way I can avoid spending disproportionately much time on UI. Most likely this will change in the future.
I think if you combined Toy Town with Medieval City Generator into one app and put it on Steam you would get lots of sales.
I agree, that it would be nice to have Toy Town as an in-built 3D mode right in MFCG to skip the whole import-export thing. That's what I planned long ago when the original Toy Town was created and I still intend to do so. But right now both MFCG and City Viewer are under active development, so at this moment it's easier for me to support them as separate programs. Another benefit of having City Viewer independent is that I can try to import villages from Village Generator.
If you add some basic NPC models... Maybe some props carts, barrels, trees... Add moving clouds that cast shadow... Add a basic screen shot mode with a focus ring and maybe tilt-shift mode with that...
As much as I'd like to play with such niceties, I guess it would be a waste of time from the users' point of view. 3D stuff is not exactly my thing and every small feature takes long time for me to implement. Actually, when I added JSON export I hoped that someone else would try making something like you described.