SPUser sUser = myweb.CurrentUser;
SPList UserList = SPContext.Current.Web.SiteUserInfoList;
SPListItem userItem = UserList.Items.GetItemById(sUser.ID);
String department = userItem["Department"].ToString();
SPListItem ListItem=myweb.Lists["UserList"].Items.Add();
ListItem["UserName"] = sUser.Name;
ListItem.Update();
see this link for more details:
How to fill a SPList using data from SiteUserInfoList?