Yes, that's exactly what GJ and CB Gallery are doing for their activity and most of the core activity. You've 2 options basically. 1 is to create a new CB Activity template by copying the default template folder below.
/components/com_comprofiler/plugin/user/plug_cbactivity/templates/default
Next rename your copy to something like "custom". Now delete every file in it except "activity_core.php". Now you can add your own custom core activity parsing code to it as needed.
The other option is to do what GJ and CB Gallery are doing and act on the activity_onDisplayStreamActivity trigger (either in your own custom plugin or CB Auto Actions), which is as follows.
Code:
$_PLUGINS->trigger( 'activity_onDisplayStreamActivity', array( &$row, &$title, &$date, &$message, &$insert, &$footer, &$menu, $stream, $output ) )
Notice most of the variables are references. They allow you to modify the message of an activity, insert new content below the message, next to the date, replace the title, or even alter the activity object variables and more.
If you do this from CB Auto Actions you will need to use a Code action then set Method to PHP. Next under Parameters within your auto action be sure to set Reference Variables to pass references for whatever variables in the above trigger you want to modify. Now you'll be able to modify them in your code with $variables (e.g. $variables would be the $message variable). As for knowing what activity entry to do this on you'd just condition against [var1_asset] under the Conditions of your auto action.