Hello,
posting an activity in a Share plugin can be easily achieved by using the DoclibActions module in Share JS code to execute an action (genericAction(obj)) or to plain submit an activity post from custom code (postActivity(siteId, activityType, page, activityData)). You need to make sure that JSON and atomentry FTL templates for your custom activity post are available in the Repository (unless you post into the org.alfresco activity namespace) and the activity-list.get web script on Share is adapted to handle custom activity feed entries.
Regards
Axel