All C code is valid C++ code; you could rename the file extension (it probably will need to be re-added to your project).
Alternatively, you could delete the C file and add however many C++ files you want. The structure of code in a static library is not nearly as strict as in other areas because it is simply a collection of object files waiting to be linked at a later point in time.
You do have a valid point though; for continuity, the static library wizard should have a language selection page...