You will need to use `contains` keyword in a syntax file (see [:help contains](http://vimhelp.appspot.com/syntax.txt.html#:syn-contains))

As an example of usage, see how the pythonDocTest syntax region [is defined](https://github.com/hdima/python-syntax/blob/master/syntax/python.vim#L347) and [used](https://github.com/hdima/python-syntax/blob/master/syntax/python.vim#L271) within a Python syntax file.