There is no valid method to create a discussion item through a workflow using create item and this would be great addition.
The activity could be Add Discussion <list> (output: <item>)
The below code is snippet that would have to be wrapped in an activity.
using (SPSite site = new SPSite("SITE URL"))
using (SPWeb web = site.OpenWeb())
SPList list =web.Lists"LIST TITLE";
result = SPUtility.CreateNewDiscussion(list.Items, "SUBJECT");
resultSPBuiltInFieldId.Body = "BODY";