While converting a Task to Case following changes made internally:
1. New CASE record is created.
2. Current TASK field [regardingobjectid] is updated to newly created CASE id.
One way comes in mind.
Create a plugin:
Primary Entity: TASK, Message: Update
Code:
if (entity == null) return;
if (entity.LogicalName != "task") return;
if (context.MessageName.ToLower() != "update") return;
//get regarding objectid
if (entity.Contains("regardingobjectid") && entity["regardingobjectid"] != null) {
RegardingRef = (EntityReference)entity["regardingobjectid"];
}
if (RegardingRef == null) return;
if (RegardingRef.LogicalName != "incident") return;
Here get Description from task and update your case