Thanks for the link!
I've done something similar to this before, but it takes a lot of time and you have to be very diligent. I'm not sure I'm going to go with this route, but just in case someone else is thinking about it, here is the general idea:
The idea is similar to the link above. You add your db to SQL Source Control (SSC), which creates a new repository/folder, then inspect the folder/file structure it created. You then take your preexisting repository and rename each file to match the same structure as the one created by SSC. Renaming does a drop/add, but the history of the file is maintained. After you have committed your changes, you should be able to instruct SSC to switch to using your repository just like the link above mentions.
This might make for an interesting project for Red Gate in the future: a program could inspect a preexisting repository and suggest what changes should be made to make it compatible with SSC. However, that seems like a lot of development work for something no one seems to be requesting.