Edit: Oops, I didn't really see that you're about to create a new project type for C++ Builder files. In that case, you have to build a language package. The Visual Studio Extensibility site should get you started. Also have a look at this more specific link.
I'll leave my old answer here for reference, because it might help people who just want to build C++Builder projects without creating a whole new project type :)
You didn't specify the version of Visual Studio, but I'll assume a recent one. In Visual Studio 8 and 9, most project files (all popular ones except Visual C++) are actually MSBuild files and can therefore be built by MSBuild. You can add a simple command line task (Exec) to build your bpr on the command line, or you can create a custom task for this (if you don't find one already available - the search terms should be MSBuild and custom task). This way, both Visual Studio and MSBuild can build anything you like. If you don't have an MSBuild file to start with or want to dive into developing a task, the MSBuild project template for Visual Studio will help you.
Oh, and other than that, if you don't actually need C++Builder things, you might as well export the BPR as a solution (or create a new solution and add the files).