I've used the following with some success.

            //get original user
            string oUser = WindowsIdentity.GetCurrent().Name;

            //impersonate application pool to impersonate processAdmin
            using (WindowsImpersonationContext impersonatedUser = WindowsIdentity.Impersonate(IntPtr.Zero))
            {
                using (Connection connection = new Connection())
                {
                    SCConnectionStringBuilder connectionString = buildConnection();
                    connection.Open(K2ServerName, connectionString.ToString());
                   
                    //now impersonate original user to action worklist item
                    connection.ImpersonateUser(oUser);
                    workListItem = connection.OpenWorklistItem(serialNumber);
                    connection.RevertUser();

                    //impersonate to update process data, normal users can't
                    connection.ImpersonateUser(ProcessAdmin);
                    Hashtable updateValues = buildHastable();
                    saveActivityData(workListItem, updateValues);
                    connection.RevertUser();
                   
                    //now impersonate original user to action worklist item
                    connection.ImpersonateUser(oUser);
                    workListItem.ActionstActionName].Execute();
                }
            }