Here's the VIM documentation on how to add to existing syntax.
ADDING TO AN EXISTING SYNTAX FILE *mysyntaxfile-add* If you are mostly satisfied with an existing syntax file, but would like to add a few items or change the highlighting, follow these steps: 1. Create your user directory from 'runtimepath', see above. 2. Create a directory in there called "after/syntax". For Unix: mkdir ~/.vim/after mkdir ~/.vim/after/syntax 3. Write a Vim script that contains the commands you want to use. For example, to change the colors for the C syntax: highlight cComment ctermfg=Green guifg=Green 4. Write that file in the "after/syntax" directory. Use the name of the syntax, with ".vim" added. For our C syntax: :w ~/.vim/after/syntax/c.vim That's it. The next time you edit a C file the Comment color will be different. You don't even have to restart Vim.
You can get there by :help mysyntaxfile-add