There’s a suggestion thread under HubSpot Ideas to implement custom Task and Log Activity types. Please go there and upvote the original post. Maybe enough responses will get HubSpot’s product team to put this change into their task queue.
https://community.hubspot.com/t5/HubSpot-Ideas/Customizing-quot-log-activity-quot-and-Task-type-in-HubSpot/idc-p/38047