Add trigger/commit character to VisualStudio Html Completion

Question

I am trying to create extension for framework in which combination of html markup and custom experessions is used. I would like to trigger and commit completion session for those experessions. Is there any easy way to add custom characters to trigger and commit completion session inside html editor in VisualStudio? I know that there is this "I implement custom CompletionCommand" way of doing it. My problem is I need to combine standard html completions on html markup but I need to add support for characters that start my custom expresions completion. So I thought it would be easier to make use of something HtmlCompletionSession already has.