Hey, you actually can make it work, I did it just today. I had a model where I had +100 features already placed and didn't want to bring everything into a new model (I had a lot of "furniture" I created in 3DS Max). What you need to do is export a shapefile for your desired extent. Then, go into your model properties and change your model extent by clicking the "Load Extent From File...". Once your model extent is changed, create a new Infraworks model with the desired extent. Now that your new model is created export the model as a .IMX file. Once exported, go back into the model you want to expand and bring in the .IMX files (you may want to erase the existing terrain, water, roads, etc. from your model). This .IMX file will bring in the roads, water, and most importantly, the terrain for your desired extent. Finally, you will want to right-click on the Infraworks provided imagery (Bing) in the data sources and click configure. When you click "close and refresh" your imagery will update to cover the "new extent" of your model.
Hopefully that all makes sense, it sounds long, but the process goes relatively quickly. Much quicker than trying to rebuild a giant model anyway. Let me know if you have any questions!
Kelby Wilkison, P.E.
Civil Engineer
Trihydro Corporation