Why would you not be able to mix? just change your version to c++14 and that means that c++11 and 14 and technically 03, 98, etc will work.
Edit: to provide more detail. The entire engine is not c++11. Just what made sense to update to c++11. When you write code, most often times older versions of the spec are still valid. For example. c++17 is now out and compilers are adding support for it. This means that if I set my compiler to use c++17 that 17, 14, 11, 03, 98, etc will all still work. I can mix code as I wish. There could be deprecated code, yes, but in my experience it isn’t so likely. I still have an app from 2000-2001 that uses c++ 98, some c++11 and I started adding some c++14 where it made sense. . I can still compile it to this day in VS 2017.