CEdit *pEt = (CEdit *)GetDlgItem(IDC_EDIT_RECV);
//方式1
int nLen = GetWindowTextLength(pEt->m_hWnd);
pEt->SetSel(nLen, nLen);
pEt->ReplaceSel("要添加的文字");

//方式2
::SendMessage(GetDlgItem(hwnd,IDC_EDIT_RECV),EM_SETSEL,-1,0);
::SendMessage(GetDlgItem(hwnd,IDC_EDIT_RECV),EM_REPLACESEL,false,(long)buffer);

//方式3
CString str="append ";
int nLength=pEt->SendMessage(WM_GETTEXTLENGTH);
pEt->SetSel(nLength,nLength);
pEt->ReplaceSel(str);

//自动滚屏
pEt->LineScroll(pEt->GetLineCount());