[nightpool]

Teams are strongly encouraged (at least if they're going to be using the standard OS/Compiler combo, Windows with gcc 3.*) to use Windriver as a development environment. However, the exact invocation you're looking for to import already existing files is File > Import > General > File System, and then select the folder you want to import from and then the files you want to import. But make sure to also read the link virtuald posted, as that will help you get started using Windriver to compile and download code. Also, this is only for new files you created, if you were just editing files they should show up automatically when you start Windriver (or, barring that, after right-clicking on the project and selecting Refresh).
And be sure to understand exactly how the existing code your normal programmer has written works. Make sure you're not duplicating work he's already done and figure out how your code is going to work with his.