Default Server Event - Connect to SharePoint

Hi,

In an activity, there is a Default Server event which connect to SharePoint to add a discussion b(y a method from a dll). The code of adding to discussion in this dll is the following:

public void insertIntoIPDiscussion(string strSPSURL, string strIPDiscussionName, string strSubject, string strText, string strPostedBy)
{
if (strSubject != "" && strText != "")
{
SPSite siteCollection = new SPSite(strSPS);
SPWeb site = siteCollection.AllWebs[""];
SPListItemCollection listItems = site.Lists[strIPDiscussionName].Items;

SPListItem item = listItems.Add();

//item["Posted By"] = strPostedBy;
//item["Modified By"] = strPostedBy;
item["Subject"] = strSubject;
item["Text"] = strText;

try
{
item.Update();
}
catch(Exception ex)
{}
}
}

It's OK except the "Posted By" is "k2service" - the user that starts K2Server. I try to change "Posted by" value but it not run.
What can I do to change this to an user I can specify? Thanks.