Nesting Worflows after the fact

What is the best approach for making an existing workflow a nested workflow in a different workflow?

e.g. I've built a workflow for a detailed process...and later on someone asks for the "overarching" workflow to be created so they can see how the original workflow fits in the "big picture" context?

I tried dragging one into a blank subprocess window (right click on activity -> create workflow model)

that makes all of the sources and sinks get messed up -- as well as it graphically becomes a nightmare.

thanks