I found some defect in work of web method addComment(string in0, string in1, RemoteComment in2) of RemoteComment class.
When I tried to set author of new comment, author was not set as I wanted, but current user:

JIRASoap.RemoteComment rc = new global::JIRA_API_Class.JIRASoap.RemoteComment();
JIRASoap.RemoteUser ru = new global::JIRA_API_Class.JIRASoap.RemoteUser();
rc.body = strBody;
rc.roleLevel = "";
rc.author = "stella";
JiraService.addComment(strSessionId, strIssueKey, rc);

It is C# source

Then I downloaded the latest version of jira-rcp-plugin (version 3.8-1) source and fix some files, to add this feature. You can see these files in attachment.
My code marked with "// THIS WAS ADDED" comment.

I hope this helps you. Thank you for good product

P.S. Sorry for my english