Dynamics CRM 2013 - Activity Custom Field

Issue: 

So there is a requirement for our Dynamics CRM Online instance to house the list of Activities from the previous system.

However, the Activities from the previous system has more fields.

This is when I discovered, custom field to Activity entity is not possible as of this writing.

Solution

Is to create a "custom entity" derive from Activity entity. Using this custom entity, you can introduce new field.