Is it possible to add a new Activity type on the Task work item in VSTS and, if so, how can I do it? Thanks!
To add activities that is also available in the Work Details page and Sprint capacity view you need to go to the Process used by the Project (Make sure you have a Process that is editable for the project) and edit a work item type that use the activity field.
You can then edit the work item field and add values to the pick list
Edit work item activity picklist