Syntax highlighting is a relatively deep subject and extending an existing highlighting scheme may get complicated depending on certain factors. I recommend you find a good reference and read up on things. One highly regarded source of information is Learn Vimscript the Hard Way which has three chapters (45-47) devoted to the topic
With that being said, I ...
Here are some examples. (Note that your keywordAs already covers the bullet, you just need to highlight it as you want.)
When the delimiters are always on the same line, this is probably the simplest way to go:
syntax match Important /!!.*!!/
But to be more flexible, allowing delimiters anywhere, use a region:
" Don't use Question; it's a `:help ...