I presume you mean like adding the C++ equivalent of writing Graphics, Print or LoadImage in Blitz Basic?
If so then this is a misconception about programming languages which you will need to learn. They are not actually keywords, they are just functions. Functions which are supplied with the language and so appear to be built into it. Basic languages call these keywords because they are often included by default. Other langauges tend to place these default functions into a standard library that you include. In Java this library is called the Java API, in C# it's .NET and in C++ it's called Standard Library (or C++ Standard Library).
So to add your own keywords you just define them as functions and then import them (which I suspect might be your next question). Is that what you were after?